Aerators, such as an airstone attached by a plastic hose to an air pump, create needed oxygen so that fish and plants can survive in the small confines of a tank. Out in the wild, water is continuously moving, flushing harmful chemicals away from fish and plants. Fish can also swim to other parts of the lake, stream or ocean if they become uncomfortable in one area. However, they can’t do this when stuck in an aquarium. In a fish tank, oxygen in the water is rapidly depleted by ammonia and nitrite caused by uneaten and decaying or fish waste. If the fish are gasping for air at the top of the tank, there is not enough oxygen in the water for them to survive for long.
According to "Freshwater Aquarium Problem Solver" (David E. Boruchowitz; 2006), aquarium filters aren't strong enough to work by themselves in a fish tank. They need tank aerators to help continuously push the tank's water over the filter media. Without the extra push to help move the water, only part of the water in the tank would get cycled through the filter. This doesn't help the fish or plants living in the tank. If the tank is overcrowded, two aerators may be essential to help have the aquarium filter work to maximum efficiency and to ensure there is enough oxygen in the water for the fish to survive. Ideally, a fish keeper should never overcrowd a tank, but accidents happen. There should be no more than one inch of fish per gallon in the aquarium in order to avoid overcrowding.
Insects and Algae
Without a water agitator or tank aerator, water quickly becomes stagnant, even if a filter is used. Stagnant water is a breeding ground for biting insects like mosquitoes. They need a still water surface in order to lay their eggs on. But when the water is continuously moving, they can’t settle down in order to lay their eggs. The moving water may knock the bugs down into the depths and drown them. Stagnant water also grows algae far more quickly than water that's constantly moving. Too much algae in a tank will absorb all available oxygen in the water and not leave any for the fish. Stagnant water also becomes cloudy, discolored and after a few days emits a foul odor.