What Are Aquarium Sponge Filters, How They Work, and Why Choose One
What Are Sponge Filters
A sponge filter works in the aquarium to filter biological wastes and byproducts, as well as providing substantial mechanical filtration of solid particles in the water. The way a sponge filter works is by drawing water through a porous sponge material. The sponge not only traps physical debris, its large porous surfaces serve as a home for colonies of beneficial, nitrifying bacteria, which filter biological waste into less toxic elements.
Pre-filters are another option, a sponge filter fitted directly to the intake of another filtration unit. While there are a few good options available in pre-filters, one can be easily homemade. Simply cut an appropriate sponge to fit snugly over your intake vent on a HOB filter, or attach to a powerhead leading to your canister filter.
I am not referencing box filters or internal filters in this series of articles on Sponge Filters, or other filter methods which can utilize a sponge, but strictly sponge filters, with the exception of the white Hagen Elite Sponge Filter and the Penn Plax Cascase Internal Filter which do also use carbon cartridge inserts. The reason I include these two is because although carbon cartidges can be utilized, many people choose to run them without the carbon. Though there is no open area for use of personal choice in media, the slots for the carbon inserts -left empty- may be suitable for the use of some other forms of filter media.
So these filters lie somewhere between normal box filters, which are designed to accommodate your choice of media, other internal filters -which may or may not utilize a sponge at all, and normal sponge filters. The design is the sponge filter, plus a carbon insert which may or may not be used.
An exclusive sponge filter, however, one which utilizes the sponge as its only filtration device, hopefully seeks to optimize the effectiveness of sponge filtration. Sponge filters are fairly easy to make on your own, and there are DIY articles in abundance on the subject. The over-all success of the sponge filter relies on a few principles, however, which should be kept in mind whether one is purchasing or making their own aquarium sponge filter.
How Aquarium Sponge Filters Work
Specifically, water is drawn through the sponge via the uplift caused by an air pump. An airstone is placed within the sponge, with airline connected to an air pump. As air is expelled from the airline and rushes up out of the sponge and through a rigid tube towards the top of the aquarium, water is drawn along with it, pulled through the sponge and being filtered in the process.
Airpumps and Powerheads
Alternatively a power head can be used. A power head is basically a submersed water pump, pulling water up through the sponge. There's more to be said about both methods.
It should be noted that most filtration systems do utilize some form of sponge filtration. Any time water is filtered through a sponge -regardless of where in the filtration process or unit the sponge is placed- beneficial bacteria will form and biological and mechanical filtration will be happening there.
Why Choose a Sponge Filter for Your Aquarium
There are only two drawbacks of the sponge filter, so let's get them out of the way before we address the many positives. The first drawback is its complete lack of chemical filtration. Chemical filtration is typically accomplished through the use of activated carbon, which absorbs chemicals in the water. A healthy, established tank -or even one started off correctly- is very unlikely to have any harmful chemicals in its water, however, and the truth is, most aquarium enthusiasts do not regularly bother with chemical filtration. They can usually rig a filter bag of carbon to a sponge filter when chemical filtration is needed, such as after medicating a fish.
Still, it is a point that should be understood. The sponge filter in no way accomodates the use of any other filtration media. While a bit of carbon can be rigged temporarily on at least some models, and the white Hagen Elite model actually is made to use carbon cartridges, generally speaking there is little to no given space dedicated to media baskets or such.
The second drawback of the sponge filter is its unsightliness. They are not pretty, and are best placed somewhat hidden within the tank. This is easily done with plants, rocks or other tank décor.
The Wet/Dry Biowheel Myth
Sponge filters tend to have a dismal reputation amongst many aquarists, despite their strength as a biological filter and their utilization of the time tested uplift principle. At the same time, sponge filters are a regular and highly praised tool amongst many of the more serious fishkeepers in the industry. Used by professionals and dedicated enthusiasts alike, but still snubbed by your average aquarist.
This may be partly due to their simplicity, they're not at all the latest high-tech, but their unpopularity is also no doubt due to a misapprehension contributed to by the advance of wet/dry filtration. Wet/dry filtration preaches the need of oxygen for successful breeding of beneficial bacteria, thus advancing the sale of wet/dry filtration systems such as the Biowheel.
However, many argue that the oxygen content in a healthy tank is quite sufficient for the task, this is proven to be true, and that wet/dry systems suffer from loss of breeding surface due to the buildup of mineral deposits. Indeed, independent testing by enthusiasts has supported the claim that sponge filters are much more effective biological filters.
Slow Flow Sponge Filters
Slower Flow Tanks, Hospital Tanks, Breeding & Fry Tanks, and Emergency Tanks
Clearly there are circumstances under which a sponge filter is hands-down the best choice of filtration. In fry tanks, sponge filters utilizing sponges with tiny pores is going to be much less detrimental to your fry population.
They're also often the preferred choice of betta keepers, or other keepers of fancier fish. Sponge filters pose no risk to their long flowing fins and tails.
The slower flow rate associated with air-pump driven sponge filters is more suitable to smaller tanks, and less disturbing to fish species who dislike strong flows, such as bettas, neon tetras and african dwarf frogs. Plant life is also less at risk with a sponge filter than with many other aquarium filter systems.
The ease of use and maintenance also makes sponge filters great for hospital tanks, breeding tanks, and other temporarily used tanks which one would not wish to have to spend too much time maintaining on a regular basis. Many fishkeepers will use pieces of existing, used sponges to seed a filter in an emergency tank.
Sponge Filter as Supplementary Filter
While many feel that a sponge filter is quite adequate an aquarium filtration on its own, especially in smaller tanks, many other aquarists prefer to use a sponge filter in conjunction with other filtration, such as a canister filter unit, HOB unit or internal box unit to utilize other types of filter media. In this way they get the best of both worlds, the chemical and mechanical filtration of the secondary filter, as well as the superior biological filtration of the sponge filter.
This also gives redundancy to the amount of filtration, providing that additional, over the minimum, boost. Back-up filtration is also provided, in case of dysfunction in one or the other or to alternate media changes. Another benefit is that one or the other can be quickly moved to set up an emergency tank whenever necessary.
Low Cost And Ease of Use and Maintenance of Sponge Filters
The ease of use is close to unbeatable with sponge filters, they are always self-starting, and there are generally no cartridges to keep up with or change. All the aquarist need do is clean the sponge every week or two, depending on their bioload.
This is done in dechlorinated water, to avoid killing off the good bacteria. Typically the sponge is removed during regular water changes, and rinsed off in removed tank water. Removed tank water will also be a good temperature for the beneficial bacteria colonies, not too hot or too cold, which can also kill off the useful bacteria.
A trick that makes this process even easier is to use a sandwich bag over your hand, or a rubber glove. Rinse the sponge out in the removed tank water, and then squeeze out. Repeat the process several times until no more residue is being expelled. That's it, really. All the maintenance a sponge filter ever needs.
Of course, the sponge will need replaced every once in a while. How often depends on the quality of the sponge, the bioload of the tank, and whether or not any of your tank critters like to nibble on it. But if one is called to change the sponge more often than every six months, it's probably a sign of a poor quality sponge. Sometimes a sponge can last for years.
The replacement sponges never cost more than a few dollars, and the sponge filter units themselves are extremely inexpensive. This adds to their appeal, the lack of need for frequent replacement cartridges not only reduces the maintenance, but saves the aquarist quite a bundle in costs. So sponge filters are a fantastic, highly effective, low cost filtration system, by themselves or as inexpensive supplement to other filtration.
Look into my follow-up articles on this subject, Best Design Principles For A Sponge Filter, DIY or Otherwise, and Buying Sponge Filters, Prefilters, and Sponge Filter Parts.