Undergravel filters offer light filtration of fish tank or aquarium water if installed correctly. Decide to use an undergravel filer before setting up the rest of the tank, as it is impossible to install when gravel or substrate is in place. Maximum effectiveness can be obtained with larger upflow tubes, strong air pumps or power heads and a small amount of maintenance. Install it correctly for long-term biological filtration that will keep the tank water clean and healthy.
Lay the plastic grids of the undergravel filter in the bottom of your fish tank with the legs pointing down. The filter should be raised very slightly off the bottom of the fish tank to allow water circulation. The hole or holes for the upflow tubes should be in the back corners. If the grids do not fit exactly, trim them with a small hand saw.
Push the plastic upflow tube or tubes into the openings in the base you just installed. Undergravel filters (UGFs) for small aquariums usually have one on a back corner. Larger UGFs may have one hole on each side for maximum water flow.
Attach a powerhead to the top of the upflow tube by pushing the conical end into the vertical plastic tube. Position the outflow port so it either points downward slightly into the tank, or is perpendicular to the walls. This will create considerable water flow inside the tank and should be used for large tanks and fish that enjoy more turbulent water.
Attach an airstone or air diffuser to one end of a thin plastic tube and the other end of the tube to an aquarium air pump. The ends of the tube simply push over the plastic ports on either end. Place the air pump near your tank where it will not be seen and will not vibrate excessively.
Drop the airstone end of the air pump tubing down an undergravel filter upflow tube when not using a powerhead. Make sure it gets way down to the bottom of the tube. When this is turned on, after filling the tank with water, the bubbles will cause tank water to be sucked down through the gravel, the undergravel filter grid and come back up the tubes to return to the tank.
Cover the undergravel filter grid with clean aquarium gravel large enough not to fall through the holes. Two to three inches of gravel is ideal for fish tank appearance and to provide enough room for beneficial bacteria to colonise. Decorate the rest of the tank as you desire. Positioning tall ornaments or plants to hide the uptake tube can make a fish tank more attractive.