Write-Up Wednesday: Undergravel Filters


An undergravel filter consists of a plastic plate with small slits that sits under the gravel (crushed coral is often used in saltwater tanks) and water is pulled through the plate by either an air stone or powerhead. The idea behind the undergravel filter is that mechanical filtration is achieved by large particles such as detritus or fish waste getting trapped in the crushed coral. Biological filtration is achieved as the water flow also provides oxygen for biological filtration to occur.

Like any mechanical filter, an undergravel filter needs to be cleaned and cleaned regularly. Cleaning is usually achieved by vacuuming the crushed coral or by pushing water through the undergravel filter in the reverse direction. All the detritus that is pushed into the water column can be siphoned out or back in the day canister filters were used to quickly filter the water.

I much prefer live rock to an undergravel filter as live rock is much easier to keep clean. Also, if the rock that is sitting on top of the undergravel filter is not moved away, then there is no way to clean that section of the filter. If you have an fish only with live rock (FOWLR) tank, you might be ok with re-aquascaping your tank every week or two. Those of us with a reef tank, I highly doubt you are interested in potentially damaging your coral just to clean a filter.

My first saltwater tank way back in 1989 sported an undergravel filter and I clearly remember the day I added 2 more powerheads to the undergravel filter as I was told that since I had more flow, I could have a lot more fish to my tank. Yeah…that didn’t work out so well. I also remember looking under the tank watching the detritus that slipped through the filter build up.

Are there successful saltwater tanks run on undergravel filters? Very likely. Would I run a saltwater tank on an undergravel filter if given the choice? No way!

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Comments for this article (17)

  • Joe says:

    Lol those are as useful for a fishtank as the stickers that pepple adhere to their mini vans as “sports car” or “no fear” lol

  • Matt says:

    Oh the early days … 😀 I remember the painstaking layering and careful adjustments to ensure you didn’t accidentally pull the pipe out of the plate! The very vague water tests that basically presented you with a pretty colour that in no way matched the chart … Happy times… 😛

  • Joe says:

    Damn internet dropped n didnt realize the first posted.

  • James says:

    Depends on the tank. I have been running a 29 gal Nano, limited fish [lawn mower blennie and Mandrin], large coral load with under gravel filter using crushed coral and 35 lbs of live rock for more than a year. Running fine, no spikes or Nitrate, Ammonia problems. Maybe, I am just lucky?

  • James says:

    I forgot to say….over the year, I have never vacuumed the coral once. Just let the filter do it’s thing. I know, I know….limited fish, limited detritus.

  • jak says:

    James- if u do regular maintenance u will be fine but if you are not one day your substrate could be disturbed and u would.lose the whole tank.. wish u luck

  • Mike says:

    The only person I know of with any cred in the hobby who is running an under gravel filter is Paul B.

  • Joe says:

    Coukd be the undergravel filter, could just be nature.

  • Victor says:

    Lol! I remember those day. I was on top of the world with my underground filter and power heads. Thank Mark for reminding me how old I am. Jk!

  • James says:

    I started before you did Mark. I had air operated undergravel filters in my fish only tank from1973 until 1994. In that time I had an Emperor angel that lived 9 years before I sold him to a fish store, a Sailfin sold at the same time had been there for 12 years and had grown from quarter-size to dinner plate-size. A Regal angel was sold after being there for 11 years and a Powder Blue Tang that I had kept for 7 years. Tomato clowns and Domino damsels spawned repeatedly. I checked back with the store that bought them and the tank and after 4 years, all were still alive. Would I set one up with an undergravel today? No. I use drilled tanks with sumps now, but the undergravel filters did work.

  • Mike says:

    Never tried an under gravel filter myself, but this made me think: what if we attached a powerhead to it in order to keep “stuff ” in suspension? (Which goes into overflow)??? Any thoughts?

  • Ralph says:

    I had good luck with running an under gravel filter back in the day I ran a power head on one tube and the pickup for my canister filter on the other tube. And only did 50% water change every 6 months when I cleaned the canister filter .It was a fish only tank with one anemone. All did well .

  • James says:

    Mike, that’s what they called a reverse flow undergravel filter. And it worked in conjunction with a power filter. It was supposed to be better. Eheim touted this method as the preferred filtration for salt water tanks.

  • Arno says:

    While a poor long term solution. They make great quarantine filters as a verry robust bio capibulities. And keep the bio active longer when uninhabited. Yes they’re a pain to break down and clean and sterilize but with successful fish they end up being more stable and less work for quarantine. .02

  • Bob Humphrey says:

    Started saltwater in 1973, ran undergravel filters in seven tanks. They would do fine for a couple of years then a massive cleaning was needed, then you had to worry about outbreaks of diseases. When wet/dry filtration came out in the 80’s what an improvement. Now run just a 150g tank with 40g sump and basically have to do little for maintenance. Regular 30g water change and dosing and tank runs great. So easy. Oh have high bioload,25 fish and 80 corals. The undergravel filter served its purpose,so I have to give it credit, it had it’s place in history.

  • George says:

    My first saltwater tank was in 1966 and about all that we had were air operated undergravel filters and the old air operated box filters from the old freshwater days.. The hobby was relatively new for the home aquairist, but we used what was available. The undergravel filters worked great for a time, but eventually were a pain to clean. Pretty much had to break down the tank and start over. Its amazing to see how far the hobby has come and the continued improvements over the years.

  • Lazaro garcia says:

    Yes,I was 6 years old and started,working at local pet store, u worked there up to high school,I still belive in the under gravel filter ,I became a wholesaler,and belive me the under gravel with cruched coral does better than the other systems incredible but true.

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