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The main purpose of a filter is to circulate the water. This leads to better aeration and higher oxygen levels for both the fish and the critical bacteria which decompose the fish wastes. The more obvious aesthetic effect of filtration is the mechanical removal of particles in the water, but this is of much less importance. Since most aquaria are stocked far more heavily than natural conditions, constant effective filtration and circulation are essential. On large tanks ( 55 gallons or more ), consider using two smaller filters in place of one large one, and place them at opposite ends of the tank for maximum circulatory effect.

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