8.5. Under Gravel Filters

Ahhhh yes the myths live on.

Aquarium equipment manufacturers and fish stores who want to sell expensive canister filters (ah, the profit motive at work!) have invented a whole host of negative points about under-gravel filters. None of these points withstands scientific scrutiny. Under gravel filters are still one of the best forms of filtration out there.

Note the brightest, most honest fish keeper in the business, Cory of Aquarium Co-op, recommends under gravel filters. Most shops do not promote them because it is a onetime low profit purchase with no return business. Air operated under gravels have no moving parts that need replacing.  I have been using some under-gravel filters for some thirty years and they are still working well. If one can get five years out of most canisters and HOBs, you are doing well. External filters frequently leak but under gravels cannot leak. Canisters frequently spring leaks.

Note that if you clean under gravels frequently, they are a pain in the butt and do not work. If you just leave them alone, they are a great, inexpensive filter with a huge bioload capacity. Put a powerhead on them and they become even better biofilters. “Old technology” like under gravels have only been superseded by our need for shiny new things and expensive “must-haves” that we do not really need.  I have them in sixteen tanks and love them.

Placidochromis blue otter
Placidochromis blue otter

Under-gravel Filters and Biofiltration

Under-gravel filters are great for biofiltration (the media volume is second only to sumps) and crystal-clear water AS LONG AS YOU DON’T CLEAN THEM BUT EVERY FEW YEARS. The brown “gunk” in the gravel isn’t fish feces or “accumulated nitrates”, it is a soft biofloc filled with beneficial bacteria and many other beneficial organisms which are very good for your aquarium (“beneficial bacteria ain’t pretty”).

The brown filter sludge in a filter is for the most part alive and not simply waste. Removing this mud does more harm than good. The purpose of the filter media is not to filter out particles from the water as is often assumed. The media serves as the habitat for a vast array of microorganisms that include bacteria, archaea, worms, ciliates, flagellates, and many others. These microorganisms live in a community that is based on biofilms. The biofilms are created by bacteria that secret extracellular polymeric substance (EPS), which is often called “slime”. The community forms a bioreactor that processes the waste and turns it into food and energy for its members, and ultimately into organic or inorganic products that are then used by plants, evaporate, or removed by water changes. It takes a considerable amount of time to establish this “filter community”; consequently, it is very important not to disturb it unless absolutely necessary.

From the Poret Foam Supplier (Swiss Tropicals):

This is a probably the most intelligent statement any aquarium products distributor has ever made.

Symphysodon aequifasciatus Discus
Symphysodon aequifasciatus Discus

By the verb “clean” above, we mean stirring up the gravel and creating a brown soup which is then removed.  We do not mean cleaning under the plates or dismantling the whole thing. There is NEVER a need to do that. The gravel of under-gravel filters will never go hypoxic so bacterial toxins are not a worry if the gravel is stirred.

If under gravels are frequently cleaned (and the term “cleaned” includes “deep vacuuming”) they do not work. But if one uses cheap food that is loaded with fillers one can clog up even an under-gravel. So if your food is under 40% protein, one can vigorously stir up an under gravel once a year or so and then remove 75% to 95% of the brown water. The brown water left behind will reseed the under gravel very rapidly. And the fish won’t be harmed by the brown gunk in the water column.

picture of an aquarium fish Nannostomus mortenthaleri
Nannostomus mortenthaleri

We’ve used under-gravel filters in typically five to fifteen very heavily stocked tanks (which included large digging cichlids) for fifty years with cleaning every two or three years. They have given us: no ammonia spikes, no build-up under the plates, no “exposed plates”, no “nitrate factory”, no “clogging”, no “disease reservoirs” and no “anaerobic dead areas” (even under décor). And we currently have very heavily stocked cichlid aquariums.

One decided advantage of undergravel filters is that the downward water flow oxidizes and removes feces and mulm above the gravel very rapidly. This makes for a much healthier and much more attractive aquarium. You just will not see feces and mulm floating above the gravel for long if there is an undergravel filter.

And then there is the mechanical problems of hang-on-back and canister filters. We’ve had lots of leaks, seized pumps and noisy pumps with hang-on back and canister filters. Sometimes they have leaked out of the box. And they only last two to five years in my experience. We’ve had several floors damaged by leaks. That is an expensive failure. We’ve had some under-gravel filters for thirty years (maybe even much longer!) with no problems what’s-so-ever and no replacements of anything.

Aulonocara Red Top Lawanda
Aulonocara Red Top Lawanda

How an Under-gravel Filter Works

Under gravel filters consist of a plastic grate or “filter plate” which lies under small gravel in the aquarium. This plate allows water to flow freely under the gravel. Water is lifted up a “lift tube” by a flow of air. This flow then draws water down through the gravel. The gravel then acts as an excellent biological filter.

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operation of an under gravel filter
operation of an under gravel filter

Operation of the undergravel filter

To move the water in an under-gravel filter you use an air pump which moves air into the aquarium through air tubes.  This air pump will blow bubbles from an air stone at the bottom of the lift tubes and the bubbles will lift water up the lift tube and into the aquarium. This then draws water down through the gravel.

And the type of air lift used is important. Many under-gravel filters use a simple hole to produce very large bubbles that rise in the air lift tube. Modify any such filter to use an air stone. The large bubbles are very noisy and throw spray in all directions when they surface.

The many small bubbles of an air stone lift much more water than the large bubbles of the hole. So under-gravel filters that use airstones are much better than holed under-gravel filters. The ideal air bubble size is about one tenth of an inch.

Champsochromis caeruleus
Champsochromis caeruleus

One can alternatively add a pump called a “powerhead” directly to the top of the lift tube. Power heads move more water than air stones and gives somewhat better biofiltration. There is no need for an air pump, air stones or air tubes with the powerhead. This makes the cost of the two systems quite similar.

This diagram shows how to use a powerhead with a under gravel filter. If there are two lift tubes use two powerheads.

Ways to operate an aquarium under-gravel filter
Ways to operate an aquarium under-gravel filter

Another idea which came from Cory of Aquarium Co-op is to put the suction end of a canister on the top of the undergravel filter tube. This is a GREAT idea. It is the equivalent of putting a powerhead on an under-gravel while it does excellent mechanical filtration for the canister. It is especially good in a tank with fry as the flow into the undergravel isn’t large enough to trap the fry or suck them into the canister pump blades.

Protomelas taeniolatius OB Red Empress
Protomelas taeniolatius OB Red Empress

Comparing Under-gravel filters to other filters gives the following:

Filter# of 3″ fish crystal clear*Media capacity cubic inchesMedia capacity cubic centimeterCostLooksLeak potential
Cartridge internal04-1065 – 165DecentNone
Cartridge Hang-on-Back05-15100 – 245GreatCommon
Airlift sponge2 – 860-2001.000 – 3.250BadNone
Sponge with pump3 – 1060-2001.000 – 3.250BadNone
Matten corner5 – 8100-1601.650 – 2.600BadNone
Bottom-to-top Hang-on-back filter10 – 20180-2603.000 – 4.250€€GreatCommon
Undergravel with airlift15 – 301000-200016.000 – 32.000GoodNone
Undergravel with pump20 – 401000-200016.000 – 32.000GoodNone
Canister filter25 – 50500-10008.200 – 16.400€€€GreatCommon
Static sump100 – 3002000-600032.000 – 100.000€€€€ **GreatSome
Fluidized sump200 – 6002000-600032.000 – 100.000€€€€ **GreatSome
* Assumes a good filter media (foam, plastic pot scrubbers or K1)
*** A DIY sump can be made quite cheaply

# of 3″ Fish*  refers to the number of fish for which this filter can give very clear healthy water. Ammonia oxidation is twenty times easier and these numbers can be multiplied by twenty if the only consideration is ammonia.

Note that under-gravel filters need to be purchased on the internet now-a-days due to the determined efforts of the profit minded filter manufacturers (Petco still stocks them). Also note that it is very feasible to buy an under-gravel filter for say a 30 gallon aquarium and install it on ANY tank larger than 30 gallons.

Note also that for some inexplicable reason the price of aquarium gravel has skyrocketed lately. Some of it is selling for $1 a pound. At three pounds of gravel for every gallon of tank that is $150 worth of gravel for a fifty gallon tank. But note that aquarium gravel is pretty standard as a substrate. Sand is cheaper but has some serious drawbacks.

A great alternative is the oil absorbent sold by Tractor Supply and some Walmarts. It is inexpensive. If one can find the rusty brown form it is  very attractive in an aquarium. The gray is less attractive. Note I have NOT had any plugging from the large amount of fines found in this substrate.

Andinoacara pulcher Electric Blue Acara
Andinoacara pulcher Electric Blue Acara

Opposition to Under-gravel Filters

Many well meaning but ill-informed commentators on social media pan under-gravel filters and very few hobbyists use them as a result. The problem is that the panning originated in the marketing departments of manufacturers and suppliers interested in selling expensive canister filters. When local fish stores realized they made more money on expensive canister filters sales then inexpensive under-gravel filters they joined in the chorus. Isn’t the profit motive wonderful?

And then social media got involved. The well meaning but ill-informed commentators on social media did the standard thing of parroting what they had heard from other posts. What the parrots failed to realize is that those posts were put there by canister manufacturers. We have documented this form of social media “influencing” by manufacturers of expensive aquarium goods.

The chief complaint we see about under-gravel filters is that it is “antiquated technology” or “old school”. Since when is something “bad” simply because it has been around for a long time? Is the wheel antiquated?

picture of an aquarium fish Neolamprologus brichardi
Neolamprologus brichardi

When we have pressed the various well meaning but ill-informed commentators on social media who criticize our use of under-gravel filters on why under-gravel filters are bad, the best they can come up with is that they are “nitrate factories”.

Then we point out that all good filters are nitrate factories. Indeed, the function of all biofiltration is to oxidize ammonia to nitrate and that one gram of ammonia nitrogen is converted to one gram of nitrate nitrogen regardless of where or how it is done (this is a basic law of physics called “conservation of matter”). They then fail to come back 100% of the time. They have no comeback possible.

Another common complaint is that the brown gunk that builds up in the gravel is very bad “dirty” stuff that is very detrimental to the aquarium. This idea that brown gunk is detrimental is the single biggest myth promulgated in the hobby. We go into that in great depth in the undergravel in depth link below (and in many other articles throughout this website). The brown gunk is beneficial bacteria and other organisms which reduce the pollution in the aquarium, not add to it.

Another common complaint is twofold, that under-gravel filters need to be cleaned frequently of brown gunk and that they don’t work. These two complaints are just sadly self-fulfilling prophecy . Under-gravel filters work far better if they aren’t cleaned. So the folks who are cleaning under-gravel filters regularly are only insuring that their under-gravel filters don’t work as well as they should.

picture of an aquarium fish Neolamprologus leleupi
Neolamprologus leleupi

There are some aquarium hobbyists who are interested in delving deep into the science and the calculations behind all aspects of the hobby. For those who are so inclined the following is pertinent:

8.5.1. Under-gravel Filters in Depth (links to aquariumscience.org)

DIY Undergravel Filters

We have designed a DIY undergravel filter for those with a DIY bent. It is not cheap but some want such a design:

8.5.2. DIY Undergravel Filter (links to aquariumscience.org)

Bottom of the Tank Matten Under Gravel Filter

Another interesting type of under gravel filter is placing a piece of charcoal colored foam across the bottom of the aquarium over under gravel filter plates. We discuss this in this article:

8.7.7. Bottom of Tank Matten Filter (links to aquariumscience.org)


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Translated in dutch by : Joost Abrahams
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