.
A Caveat
Undergravel filters have not been widely tested as a filter in the aquaculture industry, The limited testing done on gravel filters for aquaculture was “all over the map” due to the very high loading of commercial operations. So one is left with common sense and anecdotal “testing” as the only sources of information in the analysis below.
The Myths
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 “Old technology” like under gravels have only been superseded by our need for shiny new things and expensive “must-haves” that we do not really need.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 had them in sixteen tanks and loved them.
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 “brown gunk” (my term for a type of biofilm) filled with beneficial bacteria and many other beneficial organisms which are very good for your aquarium (“beneficial bacteria ain’t pretty”).
From the Poret Foam Supplier (Swiss Tropicals):
“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.”
This is a probably the most intelligent statement any aquarium products distributor has ever made.
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.
Now there is a caveat. ANY filter, including an undergravel, can be overloaded. If one over feeds a heavily stocked aquarium with a food that has low protein levels then you can create a cesspool. And NO filter, including an undergravel, will clean up a cesspool.
We’ve used under-gravel filters in typically five to fifteen very heavily stocked tanks (which included large digging cichlids) for fifty years. Every two years or so I stir up the gravel and create a brown soup. I then do a 90% water change with frequent stirring of the water to keep everything in suspension.
The under-gravel filters 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 typically had 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.
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. The thickness of the gravel should be one to two inches, 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.
.
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. The large bubbles are very noisy and throw spray in all directions when they surface. Modify any such filter to use an air stone.
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.
One can alternatively add a pump called a “powerhead” directly to the top of the lift tube. 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. I like the powerheads as they are decidedly quieter than air stones and air pumps. Also powerheads move more water than air stones and gives somewhat better biofiltration.
I use small four to five watt powerheads and I aim the duckbill defector at the surface to maximize the “choppy waves”. This “breaking the surface tension” gives very good aeration.
This diagram shows how to use a powerhead with a under gravel filter. If there are two lift tubes use two powerheads.
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 would seem to be a great idea until you realize a leaking canister will remove ALL the water in the aquarium and kill ALL the fish. Whoops.
In order to use a canister in line with an undergravel filter one must add a hole halfway up the intake tube. This way one can only drain half the water before the siphon is broken. But the size of the hole becomes critical. Too big and you short circuit the undergravel. Too small and you don’t break the siphon. So all-in-all it is an arrangement I don’t recommend.
Comparing Under-gravel filters to other filters gives the following:
# 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.
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?
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.
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
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
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
.
Return to Filters Menu
.
Aquarium Science Website
The chapters shown below or on the right side in maroon lead to close to 400 articles on all aspects of keeping a freshwater aquarium. These articles have NO links to profit making sites and are thus unbiased in their recommendations, unlike all the for-profit sites you will find with Google. Bookmark and browse!
.
Dave says
In reply to Brad ….. Any time a canister is turned off for more than very roughly 8 hours the canister needs to be opened up and smelled. If the canister contents smell really bad they need to be thoroughly cleaned and a fish in cycle done.
Brad - UK says
Hi Dave,
Regarding your discussion with Thomas about hypoxia toxins, this raises an interesting point for canister users.
When you have an extended power outage to your canister (or if you are performing protracted maintenance or aquarium migrations), you are effectively cutting the oxygen supply to the media. How long (as an approximation) do you have when that occurs?
I am trying to understand both a) how long you need to worry about canister flow not operating before toxins are formed and b) what you should do to prevent hypoxia during a fault/outage. I appreciate there will not be an exact answer for part a).
I’m unsure if you mention this in one of your sections, but if you feel worthy, could there be a point to raise and make explicit, etc? It was unnerving to hear about the issues from Thomas when the canister was ’turned back on’ after a period of no flow.
Thanks,
Brad
Dave says
In reply to Thomas …. I’m afraid the damage is done. Nothing much you can do about it now. I do need to do some research on exactly what are the toxins that form under hypoxic conditions. I know they form but I know nothing abut the nature of the toxins.
Thomas says
Hi I’ve been trying to search for info about bacterial toxins that can be caused by low oxygen levels. I’ve accidentally cut the power to my shrimp tank filter for at least a few days and the only way the filter could get oxygen is from a 15mm diameter of surface area that allows some oxygen exchange.. I’m afraid that oxygen in the filter is depleted and harmful toxins are formed. There are a lot of brown gunk in my filter. Last night I made the mistake of turning the filter back on after it being off for at least a few days and in the morning I noticed shrimp death and I’m not sure if it’s because of the bacterial toxin or if it’s caused by something else that has formed. Could you please share your thoughts on my situation and what my next step should be? Much thanks!
Dave says
In reply to Brandon …. Your concerns are well founded. Aquasoils break down into a clay like sediment which will clog the UGF plates. And the water flow through the aquasoil will put all the nutrients into the water column very rapidly. I wouldn’t recommend aquasoil with an UGF.
Brandon says
I really appreciate all this info! I am thinking about getting back into the hobby and always loved my UGFs. Really enjoying the aguascaping movement and excited to set up a zero maintenance aquarium in the 20-30 gal range. Has anyone used aqua soil in bags above a UGF w/ powerhead? My plan is to install the grates, add a 2″ (thicker?) layer of gravel, put 2-4 mesh bags with aqua soil on the gravel (to build up hills on each end of the tank) and then cover the aqua soil bags with gravel. The thought being the aqua soil will help the plants get started while I increase the number of fish over time. My concern is mainly that the agua soil will come out of the bag and migrate into the filter grates causing the system to clog.
Dave says
In reply to Shana … ANY filter, including an undergravel, can be overloaded. You are overloading an undergravel and will need to clean it frequently.
Shana says
Hi Dave I have undergravel filter in my tank. I have two giant turtles with two tin foil barbs that are just as large as the turtles. I know that the no. 1 rule of UG filters is you don’t clean them but I feel that this is based on fish-only tanks. I wonder if it’s applied to heavy-load tanks as well? I clean the gravel once a month because if I don’t, the brown gunk piles up and covers the entire substrate to the point that it’s no longer visible. It took several months by the way.
Dave says
In reply to Steve The Lee brand should fit just fine.
steve forgione says
Hi Cory, after being out of the aquarium game for many years I have recently retired and have decided to set up a 40 gallon breeder tank. I always used UGF and had great tanks. At the time I really didn’t understand the biology like I do now. I’m finding it difficult to find an UGF in any of the pet stores in my area. I have found a Lee’s brand that I can get online. It is 36″x18″ which are the exact dimensions of the 40 gallon breeder tank. I’m afraid it may be a hair too big.? All the other sizes are either too big or smaller . Is it ok to use a smaller (by about 6″) size filter or should I chance going with the 36″x18″ and hope it fits? Thank you Steve.
Ny says
I know this is an older thread and I’m no expert, but I just wanted to share my story for those undecided… when I got my first aquarium 30 years ago, I used an UGF and a powerhead. It was the readily available filer. I wanted as much of a natural tank as possible so I used only real plants. It was before aquascaping took over the world. I didn’t know any different so I put a bunch of plants right into the gravel. I did do 10-25% monthly water changes once established with a gentle gravel vac (although I was told to do them weekly- the tank just didn’t need it), and I never had a plant die. The tank went on for 10 years with the same plants thriving and needing to be cut back. I had no idea what the plants were at the time- but I now know hornwort, swords, and a crypt were among them. I only lost two fish the entire time as well- I assume due to old age. I’m not sure how that happened since the fish I had aren’t supposed to live that long. Although I had moved the tank successfully twice before, I was going to do a big move and decided to give the fish to a local fish store. Only then did I take the tank apart and clean out the under-gravel trays. It wasn’t even gross which I thought it would be, just seemed like compost dirt/worm casting type of substance that was very thin layer (the mulm). Not smelly at all. I got back into the hobby two years ago and decided to try aquascaping because it just looks so beautiful. I assumed that all the crazy canister filters and special soil were the improved way to go. For me- it is way, way too much work. The constant tinkering and messing with levels… And even with the soil, the plants did not grow as well as with the UGF because after 10 months the soil was depleted and ALL the plants started to wane. I considered CO2 but just did not want to go down that trail. Liquid ferts made algae and root tabs did some good but just did not make the plants completely happy. I resisted tearing down the tank because it just seemed so stupid to me to uproot all the plants and stress out the fish. I love the lush carpet look that these dedicated aquascapers get, but it’s not realistic for me… (and if you notice, they are usually tearing down and changing their tanks every few months. It is just a very different type of hobby.) I also have a smaller Walstad inspired tank going because I could not find a UGF and I wanted to try something else. I like it better than the aquasoil tank but I do not love the look of sand. The mulm and sand mix for me is messier looking than gravel and you still have the problem of plants being uprooted and floating around. It just never happened with the UGF and that was with snails and Kuhli loaches rooting around. I had planned to go back to UGF again in my other tank and bring back my love of the aquarium and this site is just what I needed. Thanks for showing the simple reasons why it works. I hope it helps convince people of how wonderful it is to enjoy your low-maintenance tank and have a very healthy tank with a UGF.
Dave says
In reply to Josh …. Generally you are talking about four lift tubes at 12 to 16 inches (always maximize this length). But a lot depends on the brand name design of the undergravel filter.
Josh C says
Hi. How many lift tubes, and what height, for a 75 gallon tank using air stones? I have bottom up hob filters with foam as primary filtration but I put a UGF plate in for additional filtration and backup in case power goes out and hob don’t re-prime themselves. Thanks
Dave says
In reply to Dawn … There should be no problem with a single lift tube using a powerhead and that size tank. Go for it!
Dawn O'Connor says
Hi, can i use just 1 uplift tube on a 24 x 12″ tank using a powerhead please
Dave says
In reply to Eve … Some trapped gravel won’t hurt. And having both an undergravel and a sponge filter won’t hurt anything. And the white gunk on the spiderwood is just water mold and will go away in a few weeks,
Eve Taylor says
I purchased an UGF yesterday for my 10 gallon tank, home to my Betta. My question is: will the filter still be effective if there is some (trapped) gravel under the plate? Also……..I was thinking of splicing my air stone/sponge filter into the tubing – or is that overkill?
While I have you – that white gunk on the newly purchased spider wood, is it detrimental?
Thanks!