Under gravel Filter System
It wasnt very many years ago that this form of filtration was the backbone of our hobby. Its mostly discounted now as obsolete but it has a lot of advantages. It worked by placing a plastic plate with slots on the bottom of the aquarium and covering it with a layer of gravel. Pieces of clear tubing attached to the plate created water flow either by air stone or later on by power head. The available BB (Beneficial Bacteria.) surface was large and able to handle moderately stocked tanks with ease for many years with little or no service at all. From a biological filtration standpoint it worked well. All of the fish waste and uneaten food were pulled by the water current down into the gravel and held to either decompose or be removed by vacuuming the gravel. The main downfall of this type of system was two fold. As fish loads increased the level of BB increased along with them. This tended to clog the filter plates, necessitating breaking down the aquarium. (No big thing if youre a retail outlet with a line of 45 tanks but defiantly a problem if you only have one or two tanks and they are both full of fish.) The other problem was Oxygen. All true fish extract oxygen from the water to breath. So do the BB
Water is only capable of holding a small fraction of oxygen compared to air. The more of this available oxygen that is used up by the BB: the less of it that is left over for the fish.
