I used to have an undergravel filter and got rid of it - was always having water quality problems. You will need to clean the gravel regularly every week when you do your water change. This is necessary to prevent the gravel from "channeling," where the debris collecting in the gravel blocks some passages and forces the water through channels, resulting in decreased water flow and reduced exposure to the bacteria that clean the water.
Every so often you also need to get under the filter tray and give it a hoover as over time gunk builds up under there bigtime resulting in blockages and bacteria die off if the water flow is not constant, here are the instructions
What you need
A wet/dry vacuum, preferably the bottle type with adjustable power;
A hose adaptor to fit onto the UGF riser pipe and the cleaner hose; (You may have to make this beforehand - about five minutes of your time!)
What you do:
Remove the cap and air stone or power-head from the UGF riser tube. (If you have more than one riser, plug the other one(s) up, so that water or air can’t get in)
Connect the adaptor to the riser.
Connect the other end to the wet/dry vacuum suction hose.
If you can adjust the power, turn it down as low as possible. (If you can’t adjust the power, just drill a hole in the adaptor that you can put your finger over should you need to. This will effectively reduce the power of the vacuum by drawing air through the hole. You can adjust the strength of the suction by moving your finger over the hole changing the amount of air it can draw.)
Switch the vacuum on for as long as it needs to reduce the water in the tank by 20%. (You need to vary the time according to the size of the tank, size of the riser tube, the adaptor and the power of the vacuum.)
Top up the tank to normal level with clean treated water.
Switch the vacuum on again as above, and stir the gravel while it’s running.
Disconnect the adaptor from the riser pipe.
Put the air stone and cap back into the riser(s). (You may want to change the air stone, depending on its condition. Sorry, this will cost you a few pennies if you do!)
Top up the tank to normal level again.
Rinse out the vacuum, holding your nose while you empty it!
You’re all done! The gravel, UGF and water are now clean and clear.
Repeat the procedure every 6 months to ensure the efficiency of the UGF. (Now who says that UGF’s are difficult to clean?)
