RSS Write-Up Wednesday: Undergravel Filters

MASA Admin

8 May 2007
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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 powerbeads 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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