aqbannrm.gif - 4.02 KTip of the Week'

for the week of 5/6/99

Removing an Undergravel Filter

Removing most types of aquarium filters is not a big deal: just disconnect a hang-on or canister or pull out a box filter and most of it's collection of waste comes out with it. Undergravel filters - especially those that have not been well maintained - are a bit trickier to uninstall, since they can unleash a large amount of disgusting and even toxic grunge that has been decaying in the gravel and under the filter plate.

The Test. Before attempting to remove an UGF, the aquarist should check the overall condition of the filter bed by plunging a hand into the gravel until the filter plate is touched, then scooping up a heaping handful of gravel and letting it fall back more or less into place.

If brown particles are seen and water becomes cloudy for ten or fifteen minutes, that's pretty normal and the plate(s) can probably be removed safely. First, remove the aquarium cover, then any decorations from the tank. If there is more than one filter plate, most of the gravel can be carefully pushed to one side to remove the first plate, then pushed back to remove the second. If only one large filter plate is present, move most of the gravel towards the front half of the tank, then try slowly pulling up the back edge of the filter plate. Then slowly try to slide and lift the plate - as if you were trying to remove the bottom card from a deck without disturbing the other 51. Move gently and be careful not to bury any fish! A fair amount of "dirt" will be released; wait a few minutes for it to settle, then do a partial water change (using dechlorinated water of the same temperature) and attempt to vacuum as much up as possible. If you are replacing the UGF with a power or box filter, change or rinse its filter media several times as it will no doubt plug up quickly. It's not essential to get all the mulm out, but it's certainly easiest to get most of it at this time.

If black, foul-smelling, slimy ooze is released or water takes hours to clear, the tank is already in trouble, as anaerobic decay is taking place. Toxic gases are already being slowly released from the gravel, and unleashing a lot at once could spell disaster. There are two reasonable courses of action:

  • remove all fish, completely strip the tank, and start it over from scratch, or
  • perform a series of 30% water changes over the next few days, using one of the gravel-vacuuming syphons. Repeat until gravel is clean enough to pass "The Test", then proceed as above.

After it's over... it's best to give the tank a rest for a while. Even if the overall change is for the better, some of the good bacteria that consume ammonia and nitrite will have been lost, and it's best to give them a chance to recover. Redecorate, but don't add any new fish for a week or so. Feeding carefully and spot-checking ammonia and nitrite levels are also encouraged.


"Tip of the week" appeared regularly in 1999 and 2000.

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