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17.3. Blackwater Fish

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Author : David Bogert

Published :

Time To Read :
14 minutes
Difficulty : Level 6

Excerpt :

Summary of Blackwater Fish Needs The term “blackwater” refers to soft, acidic water which is deep brown in color, like clear tea. The dark brown color is due to organic acids in the water. Many river systems of northern South America including the Rio Negro and the Orinoco have blackwater. Some streams and swamps in ... Read more

Summary of Blackwater Fish Needs

The term “blackwater” refers to soft, acidic water which is deep brown in color, like clear tea. The dark brown color is due to organic acids in the water. Many river systems of northern South America including the Rio Negro and the Orinoco have blackwater. Some streams and swamps in southeast Asia also have this blackwater. The topic of Blackwater is looked at in some depth in this article:

17.2.1. Blackwater Biotope

Contrary to popular mythology, blackwater fish like neons, discus, apistos, loaches and rams do not need soft, low pH water or perfect “water parameters”. Rather they need exceptionally clean, crystal clear, bacteria free water. This clear water typically can only be found in over filtered aquariums whose filters are greater than four months in operation.

The key for blackwater fish is to look down the length of the tank, through four or six feet of water. Are the ornaments and fish four feet away crystal clear and do they look like they are flying in air? Or is everything kind of “dull” and unclear? If it is “dull” there can be a problem.

This is a very long and boring article on blackwater and blackwater fish. It is only for real aquarium nerds like the author.

Glass Catfish
Glass Catfish

What is Blackwater?

Blackwater is not dark because of “tannic acid”. “Tannic acid” is a term which hobbyists (and some researchers!) use when they technically mean a “tannin”. Tannins are polyphenolic organic acids found in certain plants like oaks and redwoods. There are probably several thousand “species” and variations of tannic acid.

Blackwater is is dark due to humic and fulvic acids. Humic and fulvic are generic names for literally trillions of types of black and brown phenol containing organic acids which come about when plant matter decomposes. A better term for both humic acid and fulvic acid is “humic substances”. Black Kow composted cow manure is largely black humic substances.

Marble hatchetfish
Marble hatchetfish

Blackwater is very salt free. Bacteria need some salt so there are extremely low bacteria counts in blackwater. Bacteria are largely responsible for reducing organic compounds to carbon dioxide and water. Because there are no bacteria the breakdown of organics substances goes to humic substances instead of carbon dioxide and water. In turn, because there are no bacteria in the native waters of blackwater fish, the immune systems are not very robust genetically and typically can’t fight off large numbers of bacteria in the water column.

Just for the record, humic substances are harmless to bacteria and even tannic acid has to be in incredibly high concentrations (>2,000 ppm) to kill bacteria. Note tannic acid can negatively affect fish at 40 ppm. This topic is explored in more depth in this link:

12.4.1. “Natural” Medications

Symphysodon aequifasciatus Discus
Symphysodon aequifasciatus Discus

The Bacteria-Free Requirement

Fish that originate in blackwater are typically more difficult to keep than other fish. The blackwater fish from South America seem to be especially sensitive. Blackwater fish have genetics which gives them a deficient immune system response to bacteria in the water. Blackwater fish can easily die when they are put in water which is laden with too high of levels of bacteria.

Fish like rummy noses and neons tend to die rapidly in aquariums. So various well meaning YouTube video makers ascribe all sorts of “reasons” as to why they are so fragile:

  • They have been bred in such large quantities that they have a lot of diseases
  • They are inbred
  • They are overcrowded in their breeding tanks
  • They need soft acid water

These video makers are simply wrong. None of these “reasons” are valid.

Blackwater fish tend to be difficult because of long line of factors:

  • The blackwater rivers have little in the way of sodium, potassium, calcium and magnesium salts
  • Because of the low level of salts, bacteria can’t thrive in the water column.
  • Because of the low level of bacteria in the water the immune system of the fish doesn’t need to be very robust.
  • So when blackwater fish are presented with high levels of bacteria in the water they succumb to diseases, often quite rapidly.
Neon Tetra
Neon Tetra

Blackwater Fish Species

Thousands of beginning hobbyists who have bought neon tetras for their starter aquarium have had them die within a few days to a few months.

Neons are not a fish for beginners and new aquariums!

They are from blackwater and do best in very clean, bacteria-free water. To see what happens when neons are put in a new aquarium click on this link:

11.12. Neon Diseases

Rams, Acara, Uaru, Severums, apistos, neons, brightly colored tetras, cardinals, rummy noses, pencilfish and hatchet fish are from the blackwaters of Northern South America. If a corydora species originated in the blackwater it is a sensitive fish (most corydoras are white water Amazon fish and not so sensitive).

Caridina shrimp (Caridina cf. cantonensis), glass catfish, loaches, barbed sharks, the small rasboras, and flying fox are found in blackwater in southeast Asia. While they are not as sensitive as the South American blackwater fish they are still somewhat difficult.

Nannostomus beckfordi Beckford Pencilfish
Nannostomus beckfordi – Beckford Pencilfish

Beginners often put clown loaches into a new tank and then come on social media and say the clown loaches all got ich and died. They then ask why none of their other fish got sick and died. The answer is that loaches are a blackwater fish and need very bacteria free water. The loaches got a bacterial infection followed with secondary epistylis (which looks like ich) and died.

Note that while discus are technically mainly descended from species which live in the white waters of the Amazon (there is some debate about this topic), they should definitely be treated as if they were the most sensitive of the blackwater fishes. This appears to be due to the fact that their skin mucus is different than most fish mucous. This makes them very sensitive to high bacteria count in the water. More can be found about discus at this link:

17.11. Discus

Sawbwa replendens Rummy Nose Rasbora
Sawbwa replendens Rummy Nose Rasbora

How to Keep Blackwater Fish

Bacteria free water is obtained by several factors:

  • Over-filtration. Using a very good high surface area filter media in a large filter (preferably a sump but definitely not a hang-on-back filter) that gives over 100 square feet of surface area for every pound of fish.
  • Having a filter which has been established and uncleaned for at least four months. This gets the build-up of beneficial brown gunk to the point where it can be effective.
  • Adding a UV Unit. A UV unit will prevent bacteria from reproducing somewhat. But UV doesn’t remove dissolved organic compounds (DOCs) like biofiltration will. So UV should only be an adjunct to very good biofiltration.
  • Using a very low salt content water like RO water. Bacteria have difficulty growing in low salt water. This is where the often heard comment of “Discus require RO water” comes from. Note that the water must be very low in salt (less than 50 TDS). Anything over 50 TDS and bacteria can flourish. We do NOT recommend this  way of controlling bacteria as it is very easy to go too low and kill the fish.

It is difficult to understand and decidedly counter-intuitive that blackwater fish need a filter filled with “brown gunk”. How does having a lot of bacteria filled brown gunk result in bacteria free water? It would seem exactly the opposite would be true.

Rasbora kubotai Neon Green Rasbora
Rasbora kubotai – Neon Green Rasbora

Think of it this way ….. there will be fish feces and uneaten food in any tank. This material will without fail feed bacterial growth somewhere in the tank. The question then becomes ….. do you want the bacteria in the water column or do you want the bacteria in brown gunk in the filter? It is far better for the fish for the bacteria to be in brown gunk in the filter.

This concept is better analyzed in this link:

6.2. Biofiltration

Apistogramma Macmasteri
Apistogramma Macmasteri

Hang-on-back Filters

Hang-on-back filters of any variety typically won’t cut it with blackwater fish. If you have a only a hang-on-back filter you MUST add some good biofiltration somewhere in your aquarium. A hang-on-back just doesn’t have the necessary biofiltration volume even if used properly. If the hang-on-back is a cartridge and the cartridge is being changed or cleaned on week to month schedule there will literally be no biofiltration.

If one wants blackwater fish and only has a hang-on-back filter one should add a good biofilter. That can be a sump, canister, two sponge filters or an under-gravel filter (yes, under-gravel filters are an excellent filter). Just let a lot of brown gunk build up in these biofilters and never thoroughly clean them and one will be just fine.

If is one has heard all the negative myths about under-gravels here is a link to an in-depth analysis of under-gravel filters and why they work so well:

8.5. Under-gravel Filters

Rummy Nose Tetra
Heros severus – Red Spotted Severum

Poorly Filled Canister Filter

If you have a canister but it is filled with the media that came with it, like bioballs, Matrix, Biohome, ceramic rings and/or lava rock, it is a poor biofilter and won’t make the cut with blackwater fish. One needs to change the media to something like foam, pot scrubbers or static K1 media. These media have much more surface area and have been proven by actual testing to give much better biofiltration than the “normal” media. Here is a link to more about this topic:

7.1.3. Test of Filter Media

Uaru amphiacanthoides Uaru Cichlid
Uaru amphiacanthoides – Uaru Cichlid

Over Cleaning the Filter

If you are cleaning your biofilter media thoroughly more than once every four months to a year, you need to stop in order to keep blackwater fish. The brown gunk in the filter is good beneficial bacteria and shouldn’t be cleaned until the flow noticeably slows. Actual testing of a sponge filter confirmed that a thorough cleaning is a very bad thing to do to a filter. Click on the following link for some interesting test data:

6.8. Thorough Cleaning

Paracheirodon simulans Green Neon Tetra
Paracheirodon simulans – Green Neon Tetra

UV Sterilization

UV sterilization is an excellent option with blackwater fish. It kills the bacteria in the water column without affecting the bacteria in the filter, which is exactly what one wants with blackwater fish. But it does not remove dissolved organic compounds (DOCs). Only good biofiltration removes DOCs. So UV should only be used as an adjunct to excellent biofiltration. Here is an article on UV sterilization:

14.1. UV Sterilization

Clown Loach
Clown Loach

The Adaptability of Blackwater Fish

There is no reason to keep most blackwater fish (and shrimp) in acid water. Blackwater fish can easily be kept in water buffered to 7.6 to 8.3 pH with crushed coral or baking soda. They can do very well at that pH, contrary to popular myth. Indeed, I’ve seen discus in 8.3 pH water doing great.

The reason blackwater fish do well in hard water at a pH up to 8.5 has to do with salts, namely sodium, potassium, calcium and magnesium. Most fish in the world would die in the waters of the Rio Negro. They die because their bodies can’t prevent sodium, potassium, calcium and magnesium from leaching out of their bodies and into the blackwater, not because of the acidity. If one has an blackwater tank that has very soft water (technically a tank with less than 60 total dissolved solids or TDS) stock ONLY fish from blackwater.

Blackwater fish have special adaptations to deal with this low salt content. But blackwater fish do just fine in waters with more salts than the Rio Negro has. Water with moderate to even high amounts of sodium, potassium, calcium and magnesium, even with pHs up to 8.5, is fine for blackwater fish as long as the bacterial count is low.

References on this subject include: “Ion transport, Osmoregulation, and Acid-Base Balance”, Marshall, et. al. 2006, and “Ionoregulation in Tropical Fishes from Ion-poor, Acidic Blackwaters”, Gonzalez, et. al. 2008:
Cardinal Tetra
Cardinal Tetra

And, other than the bacteria free requirement, blackwater fish are NOT any more sensitive to water parameters than any other fish. They can take ammonia spikes, temperature variations, nitrite spikes and high nitrate levels just as well as any other fish. They do not need special food unless one is breeding them. They do not need some sort of “blackwater tonic” to thrive. If you are breeding them or if you are trying for show quality fish this all changes but that is true of many fish, not just blackwater fish.

It is important to note that bacteria count is logarithmic. “Dirty water” might be ten million bacteria per milliliter while “clean water” might be 10,000 bacteria per milliliter. Going from ten million to five million with a fifty percent water change is NOT going to do a lot for any fish. Contrary to popular mythology lots of water changes are NOT a panacea for aquarium fish. Over filtration IS a panacea.

Botia almorhae Yoyo Loach
Botia almorhae – Yoyo Loach

The Oscar, a blackwater fish known for its predilection to get fatal hole in the head disease in water which doesn’t suit it, is an invasive species in the hard alkaline surface waters of Florida. The author is in South Florida on a river and the only small two to four inch fish in the alkaline waters of the river are Oscars. There are millions of them.

Note there is some controversy about whether or not Oscars are a blackwater fish. Oscars have been found in rivers in South America which are not blackwater. But whether these are natives or transplants is unknown.

Astronotus ocellatus Oscar
Astronotus ocellatus – Oscar

The author has been very successful keeping blackwater rams in the hard alkaline waters of Florida. The water needs to be extremely clean and typically is very over-filtered with a strong UV sterilizer. Bacteria free water is much more important than a perfect pH.

Note that there is some research which says fish from blackwater breed optimally in a pH of 5.0 to 6.5. So if one wants to breed some blackwater fish keep that in mind. But obviously Oscars are not one of those sensitive fish.

There used to be a fish store in Venice Florida called “Father Fish”. He largely sold aquarium plants but had literally hundreds of aquariums filled with all sorts of freshwater fish in a mad jumble. For his freshwater he used reject water from his RO unit which he used to make his saltwater tanks up. Since he is in Venice (South Florida is all hard water) his water was undoubtedly very hard and alkaline.

Neocaridina davidi Shrimp
Neocaridina davidi Shrimp

And he was making it more hard by taking out RO water first. Yet he had all sorts of Amazon blackwater fish in with all sorts of African fish and all sorts of Asian fish. He had a lot of rams and appistos scattered through all his aquariums. He had very heavy stocking in all the tanks. His were some of the healthiest tanks I’ve ever seen. And he had been doing it this way for 20 years!

One thing should be noted here. Many “experts” on social media say that corydoras and other Amazon fish are sensitive to salt. This is simply a myth. A very well run study at the University of North Carolina proved that salt is not detrimental to corydoras.

“Salt Tolerance in the Callichthyid Catfish (Corydoras aeneus)”, Murphy and Lewbart, 2003:
Aquarium Fish Boraras maculatus, Pigmy Rasbora
Boraras maculatus – Pigmy Rasbora

Schooling Fish

Most blackwater fish have very bright colors and are schooling fish. This is simply because in brown water it isn’t easy to find another fish to mate with. So the brighter the color and the more fish in a school the better the chances to mate. These bright colors make them very popular in the aquarium trade. But the schooling behavior means they need to be kept in groups of at least five. This includes the very popular clown loach and the kuhli loach. Loaches are schooling fish!

If schooling fish are kept alone or with only one or two of their species, then the fish will become very stressed. This stress will often play out as heavy duty aggression. Schooling fish like the serpea tetra can become killers if not in groups of at least six and preferably ten fish.

The exceptions to the schooling requirement are the various blackwater cichlids. The male cichlids tend to fight with any rivals and can be decidedly anti-social. Discus are a notable exception and do best when in schools of at least six.

Rummy Nose Tetra
Rummy Nose Tetra