Sponge filters are by far the oldest filter design available. However, this does not mean that they are inferior. The exact opposite is true. Sponge filters are perhaps some of the most efficient filters available today. Many a seasoned hobbyist, including myself, have used countless different filters over the year, only to realise that the humble sponge filter will do the job 95% of the time.
Now, before I go any further, I would like to clarify that there are obvious circumstances in which a sponge filter is either impractical or unsuitable. The sponge filter is almost exclusively used in freshwater tanks, as it is ill-suited to a marine system. However, sponge filters can be and are used for marine quarantine systems. Besides that, tank size is the only other aspect to take into consideration. Many people believe that sponge filters are only suitable for small aquariums. While there is a limit to how much water a sponge filter can filter, sponge filters are suitable for tanks much larger than most people assume.
A friend of mine recently told me that he had all but given up on using anything other than sponge filters in his tanks. Considering that his freshwater tank measures 6ft x 2ft x 2ft and contains a variety of large South American cichlids and a variety of large loaches, fish that produce a lot of waste, this is particularly impressive. He now uses two sponge filters to filter that tank. On top of that, he seldom does water changes (think of this what you will), so the sponge filters are almost entirely responsible for maintaining his excellent water quality.
A sponge filter works by pumping air through a narrow tube that runs through the centre of the filter. When the air reaches the end of the tube, it spills out and rises back to the top through a tunnel that runs through the length of the filter. As the air bubbles rise, they displace water which in turn draws more water through the surrounding sponge. This creates a suction affect, drawing water and debris through the sponge. The large surface area of the sponge provides ample room for beneficial bacteria to grow and the excess oxygen ensures that the oxygen-hungry bacteria are not limited in this aspect.
Sponge filters can also be run by powerheads.
Why use a sponge filter?
On top of being a great filter for the general aquarium, sponge filters stand out as the filter of choice in a handful of circumstances. Many fish breeders use sponge filters almost exclusively. The reason for this is two-fold; the gentle filtration provided by the sponge filter prevents fry from being sucked into the filter, as would happen with say a power filter or canister filter and secondly, microbial colonies that inhabit sponge filters provide a beneficial secondary food source for fry.
The sponge filter is also the reigning filter of choice among those hobbyists which specialise in black water species. A community of which I am a proud member. Most black water habitats exhibit little or no water flow, so in order to replicate these conditions, sponge filters are used.
Quarantine and Hospital Tanks
Many people, myself included, use sponge filters in their quarantine or hospital tanks. This is largely due to the fact that small sponge filters can easily be placed in the sumps of display tanks or even the display tanks themselves. Then, when the need arises, they can easily be removed and placed in a quarantine or hospital tank.
Pre-cycling Sponge Filters
Keeping sponge filters in your display tanks has another benefit. When setting up a new aquarium, the cycling process can be skipped if you have pre-cycled media on hand. Sponge filters can be used as pre-cycled media to jump start an aquarium. Even if you plan on using a different filter on the new aquarium, a sponge filter taken from an already established aquarium will allow you to begin adding fish immediately. In the meantime, the main filter can be cycling.
Sponge filters are able to provide both mechanical and biological filtration, however, most do not provide chemical filtration. This is not really an issue though as when sufficient mechanical and biological filtration are used, there is little need for chemical filtration.
If you want to use sponge filters and think that chemical filtration is necessary, there are now sponge filters on the market which enable you to add chemical filtration media.
Sponge Filter Maintenance
Sponge filters are really easy to maintain. The key is simply to remember to perform the maintenance regularly. The best way to clean a sponge is to perform a water change and use some of the water that was just removed from the display tank to rinse the sponge filter.