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Using Undergravel Filters to Their Full Potential

Undergravel FilterWith all of the technological advances in the aquarium hobby, I thought it would be nice to take a look at one of the most rudimentary and long-standing pieces of equipment in the industry – the Undergravel Filter.

Undergravel filters get a pretty bad rap. They are sometimes considered to be a relic of aquarium history, but are they still serviceable? Some hobbyists swear by them and some hobbyists swear at them. How your tank runs using an undergravel filter is dependent on a couple of factors – set up and maintenance. Let’s take a look at some of the common problems associated with undergravel filters and explore some solutions.

The Problem: They don’t work well enough. Some say undergravel filters are inefficient in tanks that have a lot of fish.

The Fix: Proper set-up is very important with filtration. This is especially true of undergravel filtration. Using the proper size and amount of gravel plays an important role in how well your filter will function. To ensure maximum efficiency with your filter, you will need to make sure that you have at least 2 to 3 inches of gravel above the filter plate. The gravel should be of mixed grades, but make sure that you don’t have anything so small that can slip under or through the grating. You will also want the gravel to be evenly distributed across the bottom. Since water flow follows the path of least resistance, an area with less gravel will receive more flow than areas that have more gravel. This will result in less efficient use of the entire filter plate. Another thing to look out for is the use of large pieces of stone or wood that sit flat on the gravel. These ornaments can restrict flow over the area. 

Another recommendation: Use compressed sand airstones instead of the compressed paper ones. Paper airstones get clogged up quickly. You should change airstones out every month regardless of which ones you use, but the sand airstones will work better for longer periods of time.

The Problem: They are DIRTY! Many hobbyists have horror stories about the amount of gunk that builds up beneath the filters over the course of years. Another complaint is that they’re a pain to maintain.

The Fix: Fish waste and other detritus is pulled down through an deposited under the filter plate, so it is easy to understand why someone might consider them to be dirty. People assume that you should not siphon the gravel if you have undergravel filtration, thinking it will destroy or suck up the beneficial bacteria. However, proper maintenance will ensure that excessive waste and detritus do not build up underneath the plate.

planted displayTo keep your filtration running at peak performance you should perform weekly water changes. Mentally divide your tank into quarters. Each week, select a quarter of the tank and siphon the gravel in that section. Using your gravel siphon, go about a half inch down into the gravel. This should remove a good amount of the gunk that settles under the filter plate while keeping the bacterial colonies from being totally wiped out. It should also allow the bacteria to repopulate the area before you have to siphon the next section. By following this regimen, your undergravel filter should not accumulate excessive waste or produce nitrates in large amounts.

Simply put, undergravel filtration has been around so long because they do work. The low-tech and simple piece of equipment can be used successfully for years when maintained properly. Keep your tank stocked properly (don’t overload), don’t over feed, and do water changes on schedule. You should have the same success with an undergravel filter as you would with more advanced filtration techniques on the market today.  Give an undergravel filter a chance…with proper set-up and maintenance in can be a reliable and cost-effective tool.



  1. avatar

    Hi Craig,
    Thanks for addressing this – wonderful article; I’m been meaning to cover UG filters on thatreptileblog, they are indispensable for tadpoles and small salamanders, very useful for many others also.

    My grandfather and I were introduced to UG filters via Robert Straugham’s classic, The Salt Water Aquarium in the Home. At the time (early-mid 60’s), they were salt water gold-standards and we had great luck with exotic and native fishes, as well as messy feeders such as blue claw crabs.

    I’ve used UG as sole filtration for large tanks housing sunfishes, bass, bullheads, and in zoos for exhibits with animals that produce a good deal of ammonia, including tentacled snakes and water snakes. Water quality was excellent, as evidenced by tests and the fact that food fishes lived and fed in the tanks (tentacled snakes) until captured.

    One aquarium with native fishes went over 20 years without being broken down…it was well planted also, and nearly impossible to foul..even via deaths of large fishes.

    One commercial aquarium that I work with today` uses UG filters as sole filtration on huge exhibits housing pollack, dog fish and other large fishes.

    I agree that the key lies in those points you’ve mentioned – water changes, gravel cleaning. if set up and serviced as you describe, a UG is hard to beat.

    I’ve had some luck in removing extra detritus from below plate by attaching a siphon to hole in which the upright discharge tube rests, but fit needs to be secure. I once kept a large exhibit with mudskippers and archer fishes in great condition for many years using a reverse-flow UG with power heads – this cut down on gunk below the plate.

    Thanks again, Frank

  2. avatar

    I maintained a 55 gallon freshwater setup with two breeding pair of angelfish for 12 years without ever having to break down the system for cleaning. I operated an undergravel filter across the full length of the tank, two high flow power heads, one on each end. Under the filter plate I placed a 1/2 Inch thick piece of polyester batting (bought at the local fabric shop), this was to increase the area available for the proper growth of bacteria. On top of the filter plate was three inches of fine gravel, with two inches of medium gravel on top of that, and another inch of polished river stone on that. There was a secondary hang on back filter with carbon media for extra clarity in the water. Once every two weeks I would vacuum the surface of the river stone, gently enough that the stones were never displaced, and water was only changed out 1/4 per month, and topped offwhen needed because of evaporation. I think, when properly set up the under gravel filter is the best way to go. Had I not moved to a new home ten years ago I would probably still have this same setup.

  3. avatar

    do you recommend a particular undergravel fllter?

  4. avatar

    Hello Jennifer, Undergravel filters have been decreasing in popularity and we actually only sell one brand of them at this point, the Lee’s Economy Undergravel Filter. For the most part, all undergravel filters function the same in my opinion and I couldn’t recommend one over another in terms of effectiveness. Our Choosing An Aquarium Filter blog may be helpful to you to help choose one that works best for you.

  5. avatar

    Are there any considerations when using an undergravel filter with live plants?

  6. avatar

    Hi Lou, Undergravel filters usually don’t work with live plants because the roots of the plants can clog up the area under the filter or restrict flow through the substrate.

  7. avatar

    On certain size tanks, one can purchase an under-gravel filter that comprises of one whole plate, which covers the bottom, or two plates. Is either type, better than the other? and why?

  8. avatar

    Hello Ken, Both would function the same. It is simply a matter of manufacturer specifications and personal preference.

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