Article © Shane Linder, uploaded January 01, 2002.

The purpose of this article is to serve as an introduction to the various types of filtration available to the catfish aquarist. Each type of filtration has distinct advantages and disadvantages and we hope this article will help hobbyists choose the best type of filtration for their animal's needs. The bottom line to remember is that filters maintain water quality and do not clean water. The only way to get your aquarium water clean is to replace it with clean water. That means a water change!

There are three basic types of filtration that must be discussed before talking about specific types of filters.

Mechanical: Mechanical filtration is the removal of waste products by mechanically trapping them in a medium. Mechanical filtration, of itself, does nothing to maintain water quality. It simply traps waste and debris so that they are not left floating around the aquarium.

Biological: Biological filtration refers to any media that promotes colonization by bacteria that consume waste products. These bacteria change ammonia and nitrite, both dangerous to fish, to the less dangerous nitrate. Because these bacteria are aerobic (oxygen breathing), they perform better when exposed to higher oxygen levels. The end product of biological filtration, nitrate, can only be removed via water changes. Because these bacteria colonize every surface in the aquarium, nearly all mechanical filters double as biological filters.

Chemical: Chemical filtration refers to any compound that chemically removes waste products. Carbon and zeolite clay are typical chemical filtration mediums because their small pores trap unwanted chemicals, like ammonia, and heavy metals. This is why carbon is used in gas masks and zeolite clay in cat litter. Chemical filtration is also the best way to remove unwanted chemicals such as fish medications. In time, the pores of the medium become full and the medium must be replaced.

Filter Types

Sponge filters: A sponge filter is simply a sponge attached to an airstone or powerhead. The airstone or powerhead pull aquarium water through the sponge to mechanically trap waste. Because sponges have a massive surface area, they also double as good biological filters.

Price:	Very reasonable.
Filtration:	Very good mechanical and biological filtration. No chemical filtration.
Maintenance:	Simply squeeze the sponge out weekly in a bucket of aquarium water. The sponges last for years making this a cheap long term filtration technique.
Aesthetics:	Not so good. The sponge is difficult to hide in display tanks.
Other notes:	The sponges are colonized by all sorts of aquatic animals. Because of this, they act as a sort of freshwater refugium and are full of tiny foods for fish fry. Very young fry can often be fed by simply squeezing a sponge filter from an established tank in their rearing tank. Sponge filters, unless attached to powerheads, are safe for raising even the smallest of fry. One air pump can run several sponge filters in multiple tanks.

Corner or box filters: These filters are essentially a small plastic box that is filled with mechanical and/or chemical filtration media. Circulation is provided by an external airline that bubbles and draws in aquarium water. Aquarium floss is typically used as a mechanical medium and will also serve as a biological medium. Chemical media can also be added to the box.

Price:	Very reasonable.
Filtration:	Low levels of mechanical, biological and chemical filtration.
Maintenance:	Simply squeeze the aquarium floss out weekly in a bucket of aquarium water and replace any chemical media as necessary.
Aesthetics:	Not so good. The plastic box is difficult to hide in a display tank.
Other notes:	Not as good of a mechanical or biological filter as a sponge, but opens the option of added chemical filtration. Safe for use in fry rearing tanks and one air pump can run several box filters in multiple tanks.

Undergravel filters (UGFs): An UGF is basically a plastic plate placed under the aquarium's gravel. Lift tubes, via the current created by the use of either air stones or powerheads, pull water from the aquarium through the substrate. This causes the aquarium's entire substrate to act as a biological and mechanical filter.

Price:	Very reasonable.
Filtration:	Good biological filtration and good mechanical filtration, but with long term problems (see below). Chemical cartridges are available, but generally hold too little media to have much of an impact.
Maintenance:	Because the substrate acts as a mechanical filter, UGFs require weekly vacuuming to remove detritus from the gravel. Some detritus always remains trapped under the filter plate requiring the tank to be completely broken down once a year in order to clean under the plates. From a mechanical standpoint, you are really just hiding the waste under the gravel.
Aesthetics:	Good. The plate is hidden under the substrate and the lift tubes, usually made of clear plastic, are easy to conceal.
Other notes:	Due to the size of the slots in the filter plates, the substrate must consist of fairly large gravel. Also, rocks, decorations, and driftwood will cause dead spots where the filter can not draw water. A few rocks and pieces of driftwood can easily degrade the UGF's filtration capacity by fifty percent. UGFs also can not be used with any fish that dig. Water takes the path of least resistance and thus if the gravel is not kept even, it will bypass the majority of the substrate. The final, and probably most important to catfish fanciers, drawback is that fish have a tendency to find their way under the filter plate.

Reverse Undergravel Filters (RUGFs): A RUGF works just like a UGF but uses powerheads to push water down the lift tubes and then up through the substrate. The advantage is that sponge prefilters on the powerheads act as mechanical filtration to remove debris before it is trapped in the gravel.

Price:	Fairly expensive. The plate is cheap, but the powerheads and prefilters can be fairly expensive.
Filtration:	Excellent biological (from the sponges and substrate) and excellent mechanical.
Maintenance:	Clean the sponge prefilters weekly by squeezing them in a bucket of aquarium water. The substrate should be vacuumed biweekly and the tank broken down every two years for a complete cleaning.
Aesthetics:	Very poor. The upside down powerheads and sponges are almost impossible to hide.
Other notes:	Helps solve the accumulation of detritus under the plates. RUGFs do not solve the above mentioned problems with substrate size, aquascaping limitations or trapped fish.

Power Filters: Power filters, also called "hang on the back" filters, use a pump to pull water out of the aquarium and pass it through various media before the water returns to the aquarium.

Price:	From reasonable to fairly expensive.
Filtration:	Varies from model to model. Generally good mechanical and chemical filtration but poor biological. Some models are fitted with a wheel device that adds wet/dry biological filtration.
Maintenance:	Clean the filtration cartridges and replace chemical media per the manufacturer's instructions. A special pipe brush is available and should be used to occasionally clean out the intake pipe. The motor's magnetic impeller will need replaced every few years.
Aesthetics:	Very good with only a single uptake pipe to be hidden.
Other notes:	A very versatile form of filtration with many, many models to choose from. Stick with a known manufacturer to assure you are buying a quality item. Filter cartridge replacements make this a fairly costly method of filtration over time. Look for models that have added wet/dry filtration and/or media baskets to increase filtration and water conditioning options.

Fluidized Bed Filters (FBFs): A fluidized bed filter is essentially a plastic tower filled with small porous media. This media is colonized by biological bacteria. Because the media is kept in constant suspension, all sides are colonized.

Price:	Expensive
Filtration:	Superb biological filtration.
Maintenance:	Almost none.
Aesthetics:	Good. Only requires a powerhead or pump to be added to the tank.
Other notes:	Outstanding biological filtration for the large aquaria. Especially useful in aquariums housing large catfishes that create large amounts of waste.

Canister Filters: Canister filters use a motor to draw water from the aquarium and pass it through a canister filled with media. Because the canister is completely sealed all water is forced through the media.

Price:	From expensive to very expensive.
Filtration:	Superb mechanical and chemical filtration. Minimal biological filtration.
Maintenance:	Clean the filtration cartridges and replace chemical media per the manufacturer's instructions. The canister must be opened and broken down for maintenance and then primed before turned on again.
Aesthetics:	Good to poor. Only the intake and return tubes must be hidden, so it is easy to hide what is in the aquarium. The real problem is hiding the canister itself and this will require a special cabinet.
Other notes:	The canister itself takes up space, so this must be taken into consideration. Canister filters have relatively low flow rates and thus are not good options where the aquarist desires lots of water movement. Canister filters come with a number of media baskets that give the aquarist unequalled chemical media and/or water conditioning options. They are also extremely effective due to their construction. Perhaps the best form of filtration for heavily planted aquaria.

Non-Filters

So why address non-filters in an article about filters? Because the below are devices used to maintain water quality.

Airstones: An airstone is simply a porous device that breaks the air flow from an air pump or blower into small bubbles. Because aquaria rely on aerobic (oxygen breathing) bacteria to break down waste, higher oxygen levels in the aquarium increase the efficiency of these bacteria. Airstones increase the oxygen level by forcing water to come to the surface where gas exchange takes place. Another important advantage of airstones is that they increase oxygen levels without increasing the water's temperature because they do not have a submersed motor.

Powerheads: Powerheads have the same effect as airstones when used to circulate water. However, they are capable of moving much larger amounts of water. The submersed motor, as with all tanks that contain a motor, will warm the water slightly.

Diatom Filters: Diatom filters are used to "polish" water. The filter utilizes diatomaceous earth to form a fine filter on a screen. Water is forced against the screen and particulate waste is trapped. Particles as small as one micron can be trapped by the screen. Because the screen quickly clogs, these "filters" are used only for a short duration. They are most useful for clearing up a display tank after a water change.

Ultraviolet Sterilizers: A pump is used to push aquarium water through the device where it is exposed to ultraviolet light. This light is lethal to many pathogens, harmful bacteria, algae, and fungi. Ideal for use on quarantine tanks or with very sensitive fishes that are prone to fungal and bacterial infections. Especially useful for blackwater fishes that come from low pH environments and are susceptible to infections in the aquarium.

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