The filter unit is composed of highly porous sponge material that acts to trap solid waste and debris that passes through the compartment. Similar like a box filter, it is located at the tank bottom and the force of air pump drives the flow of water moving through the compartment. As air is supplied into the filter unit and then exits, water is pulled into through the sponge material thus, this captures and filters out the waste. The whole mechanism provides a good way to treat the aquarium water by means of providing huge surface area to support growth of beneficial bacteria, and this is important especially to reduce amount of dissolved ammonia in the water by breakdown into less harmful compound (read more about nitrogen cycle).
Other advantages of using the sponge filter is that, it is easy to operate and maintain because all you need to do for the cleaning part is to rinse the sponge. The item is also not expensive, often costing less than $10 and if you are a Do-It-Yourself kind of guy, you could even construct your own DIY sponge filter. Basically, all you need is a plastic container and some sponge to get started. Other advantages with the filter is that, it is suitable to be used in fish fry tank because the weak aquarium water flow will not cause much turbulence in the water and this will avoid sucking in the fry to the filter unit.
However, the only setback when using this filter type is that it can only treat small volume of aquarium water and with heavily polluted cloudy tank (usually those occupied by goldfish), it will choked up easily in short amount of time. Thus sponge filter is best to apply and use in mini fish tank that rarely exceeds 5 gallon. Another disadvantage concerning use of this type of filtration is that the unit will sit inside the aquarium and it will take up space and look rather unpleasant. For heavily planted tanks, using sponge filter will not come as a problem since you can easily hide it up among the dense vegetation.
Other aquarium filter type: trickle filter, fluidized bed filter, diatom filter, canister filter, external aquarium power filter