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7.5. Denitrifying Media

Many people make the claim that their nitrates went down significantly after six to ten months of using Biohome Ultimate or De*Nitrate (a smaller sized Matrix), the two main media claiming to do anaerobic decomposition of nitrate to nitrogen. The familiar refrain is: “After six months my nitrate levels had gone down from 80 to 20 in the aquarium with 25 fish”.

A year long test of these claims was set up and this test clearly proved no denitrification takes place with these media. And a thorough search of the literature showed that it is impossible for these media to work for no less than six sound scientific reasons, any one of which will prevent any denitrification.

This is a very long dissertation that is only for the real nerds among us, like the author. The problem is that proving that media cannot remove nitrates by reduction to nitrogen gas is what is known as “proving a negative”. And this is VERY difficult to do.

hybrid yellow Peacock
hybrid yellow Peacock

Test Abstract

Seachem and Biohome (the originator and distributor of Biohome is the charlatan “the Pondguru”) claim their biomedia removes nitrates from the aquarium by anaerobic denitrification. So four identical aquariums were set up with four media to test these claims over the span of one year.

Two of the media were supposedly “denitrifying media” (Biohome Ultimate and Seachem De*Nitrate[identical to Matrix™ ]) and two of the media couldn’t possibly do denitrification (plastic pot scrubbers and Eshopps plastic bioballs). As a side test two additional media designs which claim to do denitrification were added to the BioHome and De*Nitrate tanks: CerMedia Marine Pure ceramic block Bio-Filter Media and a Biocenosis basket.

Both moderate flow and very slow flow filters were set up with the media. The results showed no difference in the nitrate levels of the four tanks after one year or at any point in the year. This shows the two denitrifying media did not denitrify ANY nitrate into nitrogen gas.

chart of test of Biohome and Matrix

Note that we like to run tests which anyone can easily duplicate in their home. This is not one of those tests. The media was expensive and the trial was long. Sorry! But note that we did keep the test simple enough that anyone who wanted to can easily duplicate in their home.

picture of an aquarium fish Haplochromis sauvagei Rock Kribensis Vic
Haplochromis sauvagei Rock Kribensis Vic

Test Equipment

  • Four identical 40 gallon grow out aquariums
  • Four Sunsun Hw304B Canister Filters (9 liter volume)
  • Four 2 liter very slow flow bottle filters
  • Four media (note 12 liters is 3.2 gallons):
    • 12 liters Seachem De*Nitrate(same as Matrix)
    • 12 liters Biohome Ultimate
    • 12 liters plastic pot scrubbers
    • 12 liters Eshopps plastic bioballs
  • One CerMedia MarinePure block Bio-Filter Media 8x8x4
  • One Biocenosis Basket (laterite and kitty litter) 8x8x4
  • API nitrate test kits
  • Juvenile African Cichlids
Symphysodon aequifasciatus Discus
Symphysodon aequifasciatus Discus

Test Procedure

The four forty gallon breeder tanks were existing grow out tanks with powerhead operated under-gravel filters. The canisters were set up with only one filter media filling each. The UV lights were not used.

In addition, a very slow flow 2-liter bottle filter of the same media was placed in each aquarium to create a “low flow” filtration. The slow flow filters had one drop every five seconds delivered with DC min-pumps. This gave both medium flow and very slow flow conditions in the filtration media. Note that Biohome says that low flow filtration is not needed to do denitrification with their media.

The amount of media used was three to five times the amount of media recommended by the manufacturer for a moderately stocked 40 gallon aquarium to accomplish denitrification. The two denitrifying media selected are both the exact grades recommended to do anaerobic nitrate decomposition by the manufacturer (Biohome Ultimate and Seachem De*Nitrate).

Flowerhorn Hybrid Cichlid
Flowerhorn Hybrid Cichlid

Note that the De*Nitrate media appears identical to the Seachem Matrix filter media under a microscope, both of whom appear identical to a garden soil media called Growstone. Indeed, Seachem claims Matrix does nitrate reduction:

“Matrix™ provides both external and internal macroporous surface area. These macropores are ideally sized for the support of nitrifying and denitrifying bacteria. This allows Matrix™, unlike other forms of biomedia, to remove nitrate along with ammonia and nitrite, simultaneously and in the same filter.”

Another material said to do anaerobic denitrification is CerMedia Marine Pure ceramic block Bio-Filter Media 8x8x4. Cermedia says of its blocks:

“MarinePure is designed to be a substrate for bio-filtration, specifically to target ammonia and nitrite removal (nitrification) and to minimize nitrates (denitrification)”.

I.e. they are claiming some denitrification with their blocks. So as a “side test” one of these blocks was added to the Matrix tank.

Another product which supposedly reduce nitrates to nitrogen gas is a so called biocenosis basket. So a biocenosis basket was added to the BioHome tank.

Aquarium Fish Hemitilapia Oxyrhincha
Hemitilapia Oxyrhincha

The aquariums were decidedly “over-filtered” so the amount of “brown gunk” in the filter media never reached the point where it impeded the flow though the filter media or the flow through the gravel of the under-gravel filter. The filter media in all eight test filters was not cleaned for the course of the experiment. The under-gravel filters were not cleaned for the course of the experiment. So cleaning was not a variable.

Then each aquarium was stocked with equal numbers of juvenile Malawi Cichlids. The author’s experience is that Malawi cichlids are quite resistant to high levels of nitrate. The stocking in each tank was kept identical for the course of the experiment. Each of the four aquariums had EXACTLY the same amount of food added per day, being carefully weighed out on a kitchen scale. The food was a high protein (50%) food recommended for juvenile cichlids.

Aulonocara Peacock Hybrid
Aulonocara Peacock Hybrid

Fish were added and subtracted to the aquariums throughout the year. The number and size of fish in all the tanks was kept equal through the year.

In order to facilitate nitrate reduction the nitrate levels were deliberately kept very high, 120 to 240 ppm. Higher concentrations of nitrate will increase the rate of any denitrification taking place. Each forty gallon tank initially had 17 grams of potassium nitrate added to it. This gave each aquarium 160 ppm of nitrate to start the test.

Nitrate levels were measured every two weeks. After the measurement of nitrate a water change was done only if the nitrate level exceeded 160 ppm. All four aquariums always had exactly the same amount of water changed out each water change. Other than water changes NO maintenance was done on the tanks.

No plants were added to any of the tanks. This is very important. Many “testers” of Biohome on YouTube have plants in their tanks. Plants absorb nitrates and void the test. Lighting was kept to a minimum (only turned on during feeding) to insure little algae growth, which can also absorb nitrates.

All parameters in all the aquariums were kept as identical to each other as possible for the length of the study. This was in order to make the filter media the ONLY variable.

Aquarium Fish OB Peacock
OB Peacock

Test Method for Determining Nitrate Levels

Testing of nitrate was done at the high levels (over 40 ppm) by using dilution techniques for greater accuracy. For instance, if the level was “dark red” on the API test, ten milliliters of tank water was diluted by seventy milliliters of distilled water in a graduated cylinder and retested. The resulting level was then multiplied by eight.

The testing of the nitrate was done by doing four tests in parallel. This was to insure accuracy within the testing. Each of the four aquariums had water taken out and tested simultaneously.

For instance, four vials, each identified with their respective filter media, were filled with 5 ml. of water from the respective aquariums (or from a diluted amount of water from the aquarium in a graduated cylinder). Each of the four test samples was then run through the API nitrate test at the same time. All four tubes were shaken vigorously in one hand for one minute. All four tubes then waited the same five minutes. All four tubes are then compared to each other against a white background in a well lit area.

Aquarium Fish Red Fire Queen Peacock
Red Fire Queen Peacock


Nitrate levels rose to roughly 160 in all four aquariums over the course of one year. Note that the 160 number is not important. Some people would see 200 and some would see 120 as human eyes are not spectrometers. What is important is that the four vials had virtually identical colors at the end of the year. Indeed they had virtually identical colors throughout the entire year.

The levels did appear to go up and down through the year between 120 and 240 (fish levels varied throughout the year) but the color was always almost identical for all four test aquariums.

When the four tubes were compared side by side the consistency of the color was surprising. The differences in the gold to orange to red hue in the four tubes was very slight throughout the test. A number typically could not be assigned to the difference in hue. Normally a natural process such as nitrification will give somewhat chaotic results. The results here were very consistent and predictable.

Iodotropheus sprengerae Makokola Reef
Iodotropheus sprengerae Makokola Reef

The reason this number was identical from one aquarium to the other was simple: the amount of food put in to each aquarium was always exactly the same. Since the food is the only source of nitrogen for the formation of nitrate, the nitrate levels aquarium to aquarium could not vary unless the nitrate was being reduced to nitrogen gas. This is the basic law of physics called “conservation of matter”.

The fish of course can absorb the nitrogen into their bodies as muscle. But the number and size of the juvenile cichlids was kept the same for all the aquariums.

To test this further six other test kits were used to measure the nitrate at the two month intervals. The following test kits were used:

  • Powder test kits: Onthink, Salifert and Seachem
  • Test strips: API 5 in 1, Stript Health and Hach (Hach needs to be multiplied by 4.4)

While the test kits were all very subjective and the results test kit to test kit were different, when all four aquariums were tested at the same time by the same kit the colors shown were virtually identical in all cases. This confirms the results of the API test kit with two other chemistries and SIX test kits.

picture of an aquarium fish Sciaenochromis Fryeri, maleri island
Sciaenochromis Fryeri, maleri island

Discussion of Results

There are several Biohome videos on YouTube claiming huge reductions in nitrates over the course of a year. ALL these videos show aquariums which are heavily planted and lightly stocked. It is a well known fact that plant growth absorbs large amounts of nitrates. These videos are typically made by well meaning people with a huge amount of what is known as “experimenter bias”. They paid a lot of money for some BioHome and they unintentionally set up an experiment with no control that was predestined to “work”.

There are many reasons for nitrate levels to vary, including plant growth, bacterial biomass growth, algae growth, removal of fish, fish growth, fish deaths and variations in feeding or water changes. So “testing” via one aquarium is not a valid test. Side by side comparison is the only valid way to test any hypothesis. And NO nitrate removal experiment can be done with plants in the aquarium.

This caution about the many ways to remove nitrates also applies to a type of aquarium called a “deep sand bed”. All the YouTube videos on deep sand beds have very logical reasons for the nitrate reduction other than the existence of the deep sand bed. Deep sand beds do not and cannot do significant “denitrification”. This is examined further in this link:

14.2.4. Anaerobic Substrates

Aquarium Fish Tropheus Moori
Tropheus Moori

The Profit Motive

“Expert recommendations” and “science” do not go hand in hand due to the profit motive. In aquarium products a marketing department of a company can freely “false advertise” as much as they want, it is perfectly legal. Marketing departments are extremely effective at writing very convincing science fiction, shooting down the cheap alternative and promoting the very expensive alternative. They are also very good at making charlatan YouTube videos.

This paper was sent to the “Pondguru”, a British distributor of Biohome who has a YouTube channel which seems to be the “bible” for some folks. Note this YouTube channel got a 1% accuracy rating from the author and that was generous. The “Pondguru” YouTube videos on Biohome are two ten minute rambles about how Biohome “duplicates Nature” in the aquarium. No testing. No real science. Nothing! Pure snake oil salesmanship! And note that ALL the Pondguru videos are just pure snake oil!

The best the replies from the Pondguru and the “manufacturer” could do defending their product was to say they has many anecdotal stories from users of their products where the product worked as advertised. They did not provide ANY studies from anyone which showed their product worked. They could only say laboratories had tested their product for density, surface area and mineral content. Hardly a glowing defense.

Auloncara sp Stuartgranti Galeriya Reef
Auloncara sp Stuartgranti Galeriya Reef

Then the Pondguru suggested the work here wasn’t actually done because we hadn’t provided any photos. If anyone needs a photo of a forty gallon grow out aquarium with a Sunsun Hw304B filter underneath it message us. LOL.

The Pondguru than makes the incredulous statement that anecdotal evidence is more reliable than scientific experimentation. WOW!!!  So the experiences of a bunch of folks with planted aquariums is more real than a scientific experiment with controls. That claim simply leaves us completely speechless. Note the Oxford Dictionary definition of anecdotal: “not necessarily true or reliable, because based on personal accounts rather than facts or research”

The Seachem advice website is just as ridiculous in it’s explanation of why De* Nitrate works. Saying they don’t know exactly how it works but that anecdotal evidence from “many” users says it does do denitrification. Anecdotal evidence is simply completely and totally unreliable. Virtually all of these users probably have planted tanks and have “tested” only one tank.

Sciaenochromis fryeri OB Black
Sciaenochromis fryeri OB Black

A video posted from Kaveman Aquatics on 09/03/2022. It supposedly “proved” Seachem Matrix reduces nitrates in an aquarium. Kaveman is a Seachem “ambassador” (i.e. they pay him money). End of story.

Nitrates are a real problem in waste water treatment plants. If anaerobic reduction of nitrate to nitrogen gas via ceramic media were as simple as the Pondguru or Seachem suggest, waste water treatment plants would have been using the technology many years ago. No waste water treatment plant uses or has ever used ceramic media like Biohome or Matrix to reduce nitrate to nitrogen gas.

This should be enough evidence for anyone. But some want more. If you are still interested in being bored to death read on.

Aquarium Fish Jewel Cichlid
Jewel Cichlid

Anecdotal Evidence

Anecdotal evidence is just not reliable but some seem to think it is important. So…here’s some anecdotal evidence:

Kaveman Aquatics YouTube channel did a test of Biohome which showed no reduction in nitrate with the Biohome. The test had no controls but was well done so I include it as anecdotal evidence. Note AFTER Seachem hired Kaveman he put out a video “proving” Seachem Matrix does do reduction of Nitrates. Hhhhhmmmmm  Matrix will drop nitrates when pigs fly.

I ran a seven month experiment with my FX6 loaded with Biohome. I got zero reduction in my nitrate levels. Considering that Biohome is incredibly expensive one would think that it would work. Now I know there will be a bunch of comments like “you didn’t run it long enough” or you didn’t have enough media” or “You had too much media”. All this is just blah.. blah… blah.. The Biohome didn’t work as advertised. Period.

Kaveman Aquatics

Here are some comments made in the comments section of this website:

“I ran Matrix in my 75 gallon Eheim 2217 and my Sicce Whale 500 canister filter for 2 years in my mbuna tank and never and I mean never seen a true nitrate reduction”


I’m moderately embarrassed to say I succumbed to the marketing blurb that “If Biohome is used the nitrate is also processed into soluble nitrogen which completes the full nitrogen cycle” and invested in Biohome media to naturally reduce Nitrates in my Cichlid tank. Needless to say I’ve seen no reduction in Nitrate levels which are constantly towards the top end of the API scale. 🙁

Ben Eadir

“I kept fish for most of my life. Of course reducing Nitrate is our ultimate goals. So, I am running 4 experiments. Let me explain, first = 55 Gallons with cichlid, sump, running with Biohome Ultimate. Good result for ammonia and nitrite, but ZERO results on Nitrate. I love the concept, but this is a very expensive solution for ammonia and nitrite only. Second = Wood ship bio-reactor (sized down) for my dark water 55 gallons. Specifically setup for Nitrate reduction = Reduced 20 to 30 % of Nitrate. But, it is for black water only, will tint your water. Third = BCB basket (check it out on YouTube), pending my 3 months mark for bacteria growth. Fourth = 30 gallons, 10 Gallons sump with water hyacinth. Amazing, reducing 30 to 40% of Nitrate reduction (can go up to 50% reduction).”

Cyphotilapia frontosa Blue Zaire Moba
Cyphotilapia frontosa Blue Zaire Moba

Another piece of anecdotal evidence,

“Seems like it’s all hype with Biohome. Bought this stuff because it is the craze right now. It’s super expensive but I thought it would be worth it. The concept is awesome. Nitrate reducer?! Unfortunately it tints the water a certain bronze color. I don’t know about you but I like super clear water. Also it “sheds” sand and very fine dark colored particulates. The water tests aren’t any better than biomax. In fact biomax is testing better. So maybe I did something wrong but I’m pretty familiar with aquatics at this point. Still love the pond guru videos though. He is very good. I’m sure he is super rich by now!”

Antonio C.


“Don’t get me started on the pondguru: he offers absolutely no scientific evidence as to why Biohome is the best media. I tried it once and found it to be no different to any other sintered glass media other than it broke up after only a few months. He talks the talk and that’s about it, at the end of the day he’s just trying to get you to buy his product and pretty much slates every other manufacturer whilst doing so.”

Miranda B.
Chaetostoma joropo L 445
Chaetostoma joropo L 445

“I filled a Sunsun 3000b canister with Biohome. All 4 trays and it had no filter pads. Even after a full year of use, I never saw any nitrate reduction in my African cichlid tank.”

Gurvinder Parmar

Just wanted to say “Thank You” for a great site with such wonderful unbiased information based on science and not hype. I plan on slowly switching out the Biohome media in all my canisters with foam. All of my low tech tanks are heavily planted and heavily stocked but even with weekly 25-30% water changes my nitrates stay between 40-60 ppm. Not bad, but after running Biohome for 2 years, I’m not seeing the low nitrates as promised, even with the recommended amounts of Biohome. After further reading on your site, I’m also guilty of over cleaning my canister filters. I look forward to approaching the hobby with a fresh eye with the information that you have so kindly shared with fellow hobbyist. Thanks David!

Susan Briggs

I tried to culture anaerobic bacteria (to convert nitrates into nitrogen gas) by adding an additional slow flow filter filled with Seachem Matrix and DeNitrate (a pumice stone that promises micro pores to encourage anaerobic bacteria) and seeding with Seachem Stability. Failing with that I built a filter using a 4 liter loc ‘n loc canister filled with Matrix/DeNitrate and using a Tom Aqua-lifter dosing pump. I switched from gravel to a sand substrate, got better lighting, and switched to a heavily planted tank. However, tank nitrates continued to slowly rise enough to require weekly water changes.

Michael Vormwald MJV Aquatics

‘nough said. It is all anecdotal but some people seem to think anecdotal is important.

Aquarium Fish Julidochromis marlieri
Julidochromis marlieri

The Actual Science of Denitrification

There are designs of waste water treatment plants in Europe which do indeed reduce nitrate to nitrogen gas (“Design and Operation of Moving Bed Biofilm Reactor Plants for Very Low Effluent Nitrogen and Phosphorus Concentrations”, Rusten, 2007). These plants have established that there are several conditions which MUST be met for a biofiltration media to do anaerobic decomposition of nitrate to nitrogen gas:

  • The media would have to have large amounts of organic compounds inside the media which would remove oxygen from the water by producing carbon dioxide gas (“anaerobic” means “without oxygen”).
  • The media would have to have large amounts of simple organic carbohydrates inside the media which would act as what are called “electron donors” and allow the reduction of nitrate to nitrogen gas. In most sewage treatment plants they add methanol, glycerin or ethanol to provide the simple organic carbohydrates (some plants just use the sewage but it is much slower).
  • The media would have to somehow move the water around inside the media particle so that each denitrifying bacterium could come in contact with the many nitrate molecules it will need to function and reduce the nitrate to nitrogen gas.
  • The pores in the media would have to be large enough to allow a nitrate reducing bacteria to live inside it.
  • The media would have to be permeable to allow water to pass into it, be purified of nitrate and then pass back out of the media.
  • The media would have to be very impermeable to further oxygen permeation from the surrounding water.

If ANY of these criteria are not met the anaerobic decomposition won’t work, even if the five other criteria are present. No media has a single one of these requirements covered, let alone all of them. This point cannot be emphasized enough.

NO “denitrifying” filter media meets ANY of these six REQUIRED criteria.

One major problem here is that the last three criteria are mutually exclusive. If the pores of a media are large enough for a bacterium to live in it then oxygen can permeate the media. If water can pass into the media, oxygen can also pass into it.  The media cannot be BOTH permeable and impermeable at the same time.

Image of an aquarium fish Parachromis motaguesis
Parachromis motaguesis

And there is no flow AT ALL inside the media. So the denitrifying bacteria cannot get the many molecules of nitrate they need to survive.

Also ALL the water flow is around the media, not through the media. So there is no way for the nitrate molecules to go into the media and be reduced.

And no ceramic media has ANY organic material inside it. So no ceramic media has internal organic compounds to reduce the oxygen levels. So anaerobic conditions cannot exist inside the media. Conditions must be anaerobic for denitrifying bacteria to work.

And there is no internal electron donor organic compounds. Without a suitable electron donor the denitrifying reactions cannot take place.

The media claiming anaerobic decomposition of nitrates categorically cannot do any anaerobic decomposition of nitrate to nitrogen gas. It simply can’t happen for no less than six separate, well established scientific reasons, any one of which would prevent denitrification. But the profit margin on the products is huge so the claims will keep coming.

Haplochromis Obliquidens
Haplochromis Obliquidens

Lava Rock Does NOT Remove Nitrate

There are websites ( and which claim the following about lava rock in an aquarium:

“You only have to inspect the rock and you can see it is covered in tiny holes making it extremely porous allowing water to pass through and diffuse into the rock. So what does The Hidden Benefits of Lava Rock really mean? An anaerobic environment is created inside the rock as beneficial nitrifying bacteria consume all the oxygen in the water.  Within this anaerobic environment inside the rock, denitrifying bacteria consume the nitrate and produce oxygen and nitrogen. We all know how nitrate in the aquarium is bad news for fish and shrimp so lava rock really is the most natural and best way of removing nitrate.”

I can’t sugar coat it. This statement is pure and simple hogwash. Note that the term “lava rock” in this quote is linked to website that sells lava rock. Click on the link, buy some lava rock and the website gets a commission. Don’t you just love the profit motive?

Lava Rock that Supposedly Denitrifies
Lava Rock that Supposedly Denitrifies

Again, let me emphasize, lava rock categorically does absolutely NO removal of nitrates to nitrogen gas. It is completely and totally impossible.

Oranda Goldfish
Oranda Goldfish

Other Claims

Note there are a large number of aquarium filter media which claim to do anoxic denitrification, such as Siporax.

The structure of Sera Siporax® is open-pored and three dimensional. This structure allows the formation of very effective biofilms that grow into the pores and walls. The biofilm also breaks down nitrate (denitrification) deep within the pore structure where oxygen levels are lower. Not only nitrifying and denitrifying microorganisms grow in the biofilm but also those that break down organic matter. Therefore, the pores do not clog. The biofilm grows and shrinks according to the bioload and thus adapts to the requirements. Sera Siporax® Professional filter material cleans itself biologically.”

Sera Siporex Aquarium Filter Media
Sera Siporax Aquarium Filter Media

Siporax has a university study which supposedly “supports” their claims. The study is quoted over and over again to “support” the “fact” that media do simultaneous nitrification and denitrification. The study is “Simultaneous Nitrification and Denitrification Using Siporax Packing”. Menoud et. al., Department of Chemical Engineering, University of Sydney, NSW, Australia, 2006.

In this paper we examine a biofilm process for simultaneous removal of ammonia and NOx (nitrite and nitrate). Siporax a porous Raschig-ring type filter specifically designed and manufactured for fast colonization by micro-organisms. was used to demonstrate the feasibility of simultaneous nitrification (usually an aerobic process) and denitrification (usually an anaerobic or anoxic process) inside a single reactor. Clear evidence was found that denitrification occurred inside the internal pore structure of Siporax rather than in an anoxic zone near the outlet of the reactor. The reactor was operated continuously and maximum nitrification and denitrification capacities of 0.61 and 0.83 g N/( respectively were observed in this study.” 

This study appears to use some very clever word smithing to lead one to conclude Siporax does anaerobic decomposition of nitrate to nitrogen gas without actually ever making that claim. This study put a “feed” of glucose, ammonia and nitrate into a column of Siporax and measured the nitrogen that came out of the column. They found less nitrogen in the effluent than was in the feed and “concluded”  “denitrification” was taking place. Note they appear to have very carefully crafted their words to say “denitrification” but to NOT say “denitrification to nitrogen gas”.

They seem to be very careful about alluding to anaerobic denitrification but never claiming anaerobic denitrification. They say: “denitrification (usually an anaerobic or anoxic process)”. Interesting! More very careful word crafting.

Cribroheros alfari
Cribroheros alfari

This study feeds more glucose (i.e. carbon) in than nitrogen by feeding sugar into the system. It is a well established fact that a high carbon to nitrogen ratio will result in high amounts of aerobic “assimilatory denitrification” where the brown “gunk” absorbs the nitrate into proteins in its structure. This study simply cleaned the brown gunk off when it clogged up the Siporax.

“In both columns, clogging occurred after a period of 40-45 days. In the second set of experiments, the column was washed out. Two litres of water, introduced from the outlet of the reactor, were found necessary to unclog the 0.4 litre reactor. After washing, the column was found to return to its previous performance level within six days.”

They didn’t measure how much nitrogen was tied up in the brown gunk they cleaned off. HHHmmmmmmm  This was the only study I found out of hundreds of various aquarium studies that used sugar with ammonia as a feed for a filter. One has to ask, did the authors of the study know just how much sugar to add to create the optimum assimilatory denitrification?

Aulonocara Turkis
Aulonocara Turkis

Now technically this study never makes any claim which is false. That is because technically the study never claims the “denitrification” seen in their study was a reduction of nitrogen compounds to nitrogen gas or even that the denitrification was anaerobic.  The words in the study appear to have been very carefully word crafted. The basic claim in this study was:

“Overall, this study demonstrates the feasibility of simultaneous nitrification and denitrification with Siporax. It was clearly demonstrated that denitrification occurred inside the pores of the Siporax rings”.

“Denitrification” did occur inside the rings. The denitrification was NOT anaerobic denitrification to nitrogen gas and was instead aerobic assimilatory denitrification to proteins in the floc.  But all the folks on social media quoting this study assume this study is about denitrification to nitrogen gas. And the study seems to be designed to lead one to that conclusion without actually making that conclusion. Interesting!

How to Do a "Directed" Study
How to Do a “Directed” Study

As as scientist I have to be honest. This study and studies like it I find very disturbing. This “study” would seem to be deliberately designed to lead one to a wrong conclusion. For university researchers to do this is, in my opinion,  highly unethical.

Cynotilapia Afra cobue Albino
Cynotilapia Afra cobue Albino

There are several other studies which come to similar conclusions (Koch and Siegrist [1997], Pochana and Keller [1997], Semon et al. [1997]). All these studies prove assimilatory denitrification, not anaerobic denitrification to nitrogen gas.

We did a series of studies on anaerobic decomposition similar to this study. We very carefully retained all the brown floc and digested the brown floc for three months using heavy aeration. At the end of the digestion all the nitrogen added to the system was accounted for. No nitrogen had been removed as nitrogen gas. For more on this test go to this link:

14.2.4. Anaerobic Myth

We can’t test all of these media and these claims but they are all scams and categorically do not do any denitrification. They also ALL do very poor ammonia oxidation and water clarification.

Lethrinops intermedius
Lethrinops intermedius

Removing Nitrates with Assimilatory Denitrification

Now this “assimilatory denitrification” can be used to remove nitrate from the water. One such set up would be an aquarium with three large powerhead operated sponge filters and three layers of 20 or 30 ppi foam in a large canister filter or three layers of foam in a sump. This aquarium is set up with fish and run for two months. Every day add food and a carbohydrate such as sugar daily at about one third to one half the level of the food added daily.

Then, at two months of operation, ONE of the filters is thoroughly cleaned of  the nitrogen filled brown gunk in the sponge. At three months a second sponge is cleaned. And at four months a third sponge is cleaned. The process is then repeated. This means a sponge will operate for three months between cleaning.

This will ONLY work with a very large amount of sponge per gram of fish. If there is not a large amount of sponge the water will become filled with bacteria and very toxic for the fish. I do not recommend this for denitrification. The process is far more dangerous than the nitrate is. But if one is interested more about this can be found at the following link:

8.9.3. Assimilatory Denitrification

Note that there a huge number of people who come on social media with statements like “Biohome dropped my nitrates by 60%“. If this person is queried, one will always find that they had a lot of assimilatory denitrification by any one of several mechanisms:

  • Cleaning their filter media.
  • Growing plants
  • Detritus accumulations
  • Algae consume the nitrate
  • Fish consume the algae
Panaque sp. Emperor Pleco L204
Panaque sp. Emperor Pleco L204

Other Studies

We have run across a whole host of studies of biofilms which have identified species of denitrifying bacteria in many biofilms in the aquarium and elsewhere. Many on social media quote these studies as proof that anaerobic denitrification takes place in aerobic conditions. This conclusion is in error.

Denitrifying bacteria are “facultative anaerobes”. The term “facultative” means these organisms can grow EITHER under aerobic or anaerobic conditions. They are NOT “obligative anaerobes”. They grow BETTER when oxygen is present as heterotrophic organisms, feeding off of “normal” food in the water column and multiplying. When they do this they do NOT do ANY denitrification. So of course they are found in all biofilms.

Aquarium Fish OB Peacock
OB Peacock

Belief Perseverance Effect

Now all this experimentation and discussion will fall on many deaf ears. When people have invested a lot of money in a product they tend to be very firm in their belief that the product is working and working well. This reluctance to accept the facts and the science is called “belief perseverance effect” and is something that is pointless to fight.

Nothing dies harder than a lie that people want to believe.


To illustrate this belief perseverance effect, one popular YouTube maker did a piece on Biohome. When we commented with the results of this test he said in one comment that he believed the anecdotal evidence over the experimental evidence. Then he went on in another comment to say the experimental evidence given here wasn’t peer reviewed and independently verified so it was invalid.

So he’s willing to believe the anecdotal “testimonials” of people using one tank filled with plants, but he won’t believe a controlled experiment without plants and with controls? WOW! Now that is belief perseverance at its ultimate. Note the Oxford Dictionary definition of anecdotal: “not necessarily true or reliable, because based on personal accounts rather than facts or research”

Parathelphusa pantherina Panther Crab
Parathelphusa pantherina Panther Crab

Then this same video maker went on to say that in a lightly stocked tank with Biohome as the filter media they had reduced their nitrates. Indeed “at the weekly water change nitrates had consistently been running 20 ppm, so Biohome worked“. This is a direct quote! LOL. They were doing weekly water changes, so of course the nitrates were low. It had nothing to do with the Biohome.

Subsequent videos by this YouTube video maker made it rather obvious it was not “belief perseverance effect” that was the motivation. Rather it was probably the good old “I’ve been paid xx thousands of dollars to endorse this product so that’s my story and I’m sticking to it” paradigm. Which is fine, we all have to make a living. Subsequent to the exchange in social media the YouTube video maker created a channel where he was selling Biohome.

Fake Facebook Accounts

There are also fake Facebook accounts set up by some manufacturers. These accounts post fake “does XXXX denitrifying media work?” posts followed by many glowing testimonials that “yes, it reduced my nitrates by 80%!”. LOL. They also vociferously attack anyone telling the truth about these products. This is unfortunate but is just part of the new “profit-driven” internet.

Peacock Red Hybrid
Peacock Red Hybrid

Easiest Way to Reduce Nitrates

The simplest and cheapest method of removing nitrates is simply good old water changes. If one lives in an area where water is very limited, and one can’t do lots of water changes, the best alternative for removing nitrates is aquaponics, i.e. plants. This can clean huge amounts of water of nitrates in a multitude of low-tech designs. It is important to realize that aquaponics requires huge amounts of plant for small amounts of fish. if interested this topic is explored in depth in this link:

6.6. Aquaponics

Heavy planting with light fish stocking will keep down nitrates. Plants are NOT effective with heavy stocking.

L333 Plecostomus
L333 Plecostomus

Nitrate isn’t That Toxic

Also nitrate is simply not very toxic to ADULT fish. António et. al. 2017 found no long-term effects from a level of 440 ppm. Monsees et. al. 2016 found the lower long-term damage level for adult cichlids to be 2,220 ppm. Science Direct and Semantics Scholar have several papers available, all of which say a level of 440 ppm does no long-term damage to adult fish. A recent study exposed young salmon to 440 ppm nitrate for 8 months and they grew just as well as salmon kept at 1/10th that level.

Adults of higher order animals like fish (and humans) have evolved metabolic pathways which neutralize nitrate quite efficiently.  Camargo et. al. 2004 found that crustaceans (shrimp and crayfish), amphibians (axolotl) and fish eggs and larvae are much more sensitive to nitrate and should have water with less than 44 ppm nitrate.  Note that a scientist’s 1 mg/L NO3-N translates to a hobbyist’s 4.4 ppm NO3, thus the 44 and 440 figures.

For more on the topic of anaerobic denitrification click on the following links:

14.2.4. Anaerobic Substrate

8.9. Anaerobic Reactors

Aquarium Fish Protomelas taeniolatius Red Empress
Protomelas taeniolatius Red Empress

Aerobic Performance of Biohome and Matrix

Note that the ability of Biohome and Matrix media to do aerobic oxidation of ammonia to nitrate was tested in another test. These two media did not do very well. More data on this can be found in these links:

7.1.3. Test of Aerobic Filter Media

7.2.11. Ceramic Media like Biohome

7.2.12. Matrix