Category:Freshwater Filtration (Aquarium Workings)

The purpose of filtering a fish tank is to remove toxic chemicals and otherwise undesireable substances from the water to ensure the health and growth of the tanks inhabitants both animal and plant.

Broad Types of Filtration
There are three broad types of filtration: 1) Chemical; 2) Mechanical; 3) Biological.

Chemical Filtration
Almost all fishkeepers will have to start here. Most water sources available to aquarists will contain chlorine, chloramine, both or perhaps even some other poisons. These toxins are put there by municipal water suppliers to keep bacteria out of drinking water. Exposure to these chemicals is not lethal to humans but to a fish exposure can result in death in just a few minutes. Ask your local pet store about water conditions at your tap and they can give you a product that will neutralize all the toxins commonly found in tap water.

Other forms of chemical filtration include: Activated Carbon (charcoal): Can remove some smells and discolorations from the water. Typically is not effective much past a week of use and must be replaced for maximum effect. Generally, though, aquarists replace carbon only every month for convenience as well as economic reasons. Running carbon is not required. Ammo Chips: A product similar in appearance to carbon except that it is white. These rocks scrub ammonia from the water. Duration of effectiveness depends on the amount of ammonia in the water. When full the ammo chips can be recharged by        following the directions provided with the product. Use of ammo chips is not required. Other scrubbers: Other products are available to aquarists to remove nitrates, phosphates and maybe even some others this aquarist is not aware of. Use of these filters is wholly voluntary and based on what you desire for your tank. If you are unsure ask your local pet store.

Mechanical Filtration
A mechanical filter is different from both chemical and biological filtration. Chemical and biological filtration deal with the removal of substances dissolved into the water. Mechanical filtration deals with the stuff that you can see with your naked eye like detritus and fecal matter.

A mechanical filter can have two purposes depending on the specific type of filter you are using. Pupose number one is to remove the offensive particles from the habitat so that you and your fish may enjoy your tank more -- think of it as garbage collection. Purpose number two is an extension of purpose number one in that the filter not only removes the solid wastes from the tank but traps it so that it doesn't enter the biological filter. The effectiveness of a biolocial filter is reduced by the presence of solid waste.

Biological Filtration
The purpose of a biological filter is, generally speaking, to facilitate the nitrogen cycle.

Fish produce ammonia as waste. Also, the breakdown of organic matter in the fish tank produces ammonia. Ammonia is highly toxic to fish. During the nitrogen cycle ammonia is oxidized by bacteria into nitrite which in turn is oxidized by bacteria into nitrate. Nitrite is just as toxic to fish as ammonia but the end stage, nitrate, is not nearly as toxic.

Generally speaking a biological filter is any "home" for the bacteria that perform the nitrogen cycle, though there are other forms of biological filters.

Other forms include: Plants, which us ammonia and nitrate directly as food; And, algae which also consume ammonia and nitrate. Plants and algae also remove other substances from the water that may be toxic to fish.

Specific Types of Filters
Undergravel Filter: The old faithful of aquariums; the substrate of the tank is used as both mechanical and biological filter with chemical filtration coming in the form of a carbon pack on the uplift tube. Air is pumped into the uplift tube and as a column of mixed air and water is lighter than a column of just water, the air-water mix is literally pushed up and out of the lift tube by atmospheric pressure on the surface of the tank water. As air is        constantly introduced into this tube, a reasonably steady flow is established. Water rushes through the substrate in order to replace the water that has been pushed up the uplift tubes. As the water goes through the substrate it is filtered. On the plus side it is cheap, easy and works. On the downside solid waste builds up in       the primary biological filter media and needs to be removed regularly (generally every         week). Also, chemical filtration is extremely limited in terms of both choices and capacity. Powered Undergravel Filter: Alternately, electrically powered powerheads may be used to lift water up the tubes, serving the same function as the mixed air-water in the air-driven original format. The same cosiderations for removing trapped particulate waste apply, as they did in the air-driven format, but are even more critical, as the powered form tends to move more water per unit time, and thus contributes more particulates.

Reverse-Flow Undergravel Filter: A somewhat more sophisticated format uses reverse flow by powerheads to drive prefilterd water down the tubes. The prefilter before the drive unit of the powerhead removes debris before it        goes down the tube and out under the undergravel plate or tubes such that the debris removal chore is transfered to simple rinsing of the prefilter, again frequently. This format (depending in part        on the porosity of the prefilter) removes much to most particulate matter, so protects the bio- filtration by the bacteria attached to the gravel from suffocation. The finer the prefilter, the better the particulate removal. But the finer the prefilter, the more frequently it must be rinsed to avoid clogging and reduced flow. This format does remove all gross water movement in the water column along with all surface distruption by air bubbles in the first case, or by surface ripples or breaks in the second case. With heavy stocking or current-liking fish, alternate sources of water movement may be needed.

Canister Filter: This type of filter sits Inside or below your aquarium pulling water into itself by means of an         electric motor turning an impeller. The water is filtered in stages as it is pulled through. First the water goes through a mechanical filter to remove any large particles. Next, the water is treated in the biological chamber. Finally, chemical filtration occurs. On the plus side options regarding filter media are many and mechanical filtration happens appart from biological filtration. On the downside these filters can be very expensive and require some effort to keep clean and maintained.

Back Filter: These filters hang on the back of your aquarium, hence their name. Water is pulled via electric motor running an impeller. Water is first mechanically filtered in a prepacked media that you purchase every month or so. Inside this prepacked media there is usually a chemical filter in the form of carbon. Finally, most back filters made now have some area where biological filtration takes place. On the plus side these filters are much cheaper than canister filters and have seperate areas for mechanical and biological filtration. On the downside, you have to buy a       replacement cartridge every month at least. Also, the mechanical filter tends to clog very quickly and the water that is meant to be filtered simply overflows back into the tank never being filtered at all.

Fluidized Bed Filters: This biofiltration technique uses small particles of uniform size as a substrate for the attached growth of biofiltration or cycle bacteria. These particles are suspended by the steady flow of prefitered water. The prefiltration removes depris which could distrupt the smooth flow of the water and cause the particles with attached bacteria to clump and fall out of suspention, or worse, be carried by the flow out of the filter. The particles may be fine sand, ceramic, plastic, or other material providing a bacteria-friendly attachment surface. These filters can do their job quite well, but depend greatly on the hobbyist being very conscientious about maintaining the prefilter to keep flow in the correct range. These units costs are often intermediate between back filters and canisters. This type of filter, along with the Wet-Dry filter (discussed below) are the most responsive to changes in tank bioload in this hobbyist's experience. This may be due to the excellent oxygenation and nutrient delivery of these two types of filters.

Wet-Dry Filters: These units are most often separate from the display tank, commonly below the display (as are        canisters). They are connected to the tank by drilled bulkhead fittings through the tank wall, by over-the-back siphon tubes (similar but not identical to a back filter or canister), or less often may be above the tank. The tank connections are not as simple as the other filter formats, as constant-level siphons, standpipes, or overflows are normally used. Prefilters in some form are used for particulate capture prior to the biofltration area, as they are in other formats. The biofiltration area itself is not submersed as it is in other formats, thus it is called dry even though it is constantly bathed in circulating water from the tank. The area is stocked with structures suitable for the attachment of cycle bacteria, which may be spheres or other shapes of plastic or sponge, ceramics, or any other suitable surface materials. The constant flow of        water provides the nutrients needed, but the thin films of flowing water allow access to         atmospheric oxygen, and similar off-gassing of bacterial carbon dioxide. This means that normal tank nitrification does not use up tank water dissolved oxygen, nor contribute as much to the carbon dioxide dissolved in the water as true submerse filter types do. In large and/or heavily stocked tanks, this may be a valuable consideration. The prefilters for particulate capture require the same frequent rinsing as in other formats. Note that this does not involve any disturbance to the biofilter portion, which operates for quite long periods without disturbance. Water is normally returned to the tank by a pump. Some setups have water pumped to the filter rather than back from the filter. Most often these are units above the the display tank, with passive or gravity tank returns. These systems are not inexpensive, but can handle large water volumes and heavy bioloads quite well.

Other types of filters including plant or algae beds are not often commercially available, and perhaps some others these aquarists are not familiar with.