02-27-2010, 12:46 AM
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For a sponge filter, you bury the base in your substrate, then you need to buy an air pump, some airline tubing and an airstone. You connect the tubing to the pump and the airstone (ideally with a check valve in the air line to prevent water from siphoning back into the air pump, which would break it and/or drain your tank slowly). You drop the air stone down into the filter and turn on the air pump. Basically, air bubbles are forced down the air line tubing and out through the pores in the air stone. The air bubbles then float up through the center of the sponge filter and up to the surface of the water. The moving air basically creates a weak water current. Water gets sucked through the sponge material, which provides oxygen to bacteria that begin to colonize it. They're pretty effective biological filters but don't do much in terms of mechanical filtration as the water flow is really weak.
Honestly I'd suggest an internal filter for two reasons: 1) Although sponge filters disturb the surface less than HOB filters, the bubbles still do disturb it some, which isn't an issue with internal filters and 2) internal filters offer better mechanical filtration than sponge filters. Also, once you factor in the air pump, sponge filters aren't quite as cheap as they seem. The Tetra Whisper in-tank filters are pretty good.