Devices for water purification in the aquarium – filters.
Classical filters are divided into internal and external (canister). Internal filters are placed directly inside the aquarium, while external ones are located outside the aquarium and connected to it via special hoses. Since external filters are not limited by the aquarium volume, the volume of filter media in them is usually larger compared to internal filters. And the larger the volume of filter media, the greater the biological load the filter can handle; that is, as a rule, external filters are designed for a larger biological load than their internal counterparts.
According to the type of purification, they are usually of two types: mechanical and biological. Water filtration in fishkeeping generally implies two processes: mechanical removal of suspended particles from water and biological purification of water from metabolic products. In mechanical filtration, the water flow passes through some fine-pored material, which traps relatively large, insoluble particles of dirt, organic matter, and aquarium debris. What is actually filtered out in the aquarium? Since fish excrement and food leftovers are a source of toxic ammonia, the best way to prevent rotting is to remove them from the aquarium.
The primary target is suspension—small and medium-sized particles of various origins floating in the water column and impairing its transparency. However, even the simplest filters that perform only mechanical filtration also have a biological impact. If fallen leaves clog the filter, the solution is simple: it is enough to attach a thin, coarse-pored household sponge to the intake port of the device. This makes it easy to avoid clogging and the need for regular washing of cartridges and the filter head, while rinsing the sponge is extremely easy.
Biological filtration is the most important type of filtration, resulting in the conversion of toxic ammonia into non-toxic nitrates. This is carried out by bacteria. During biological 
filtration, the aquarium filter performs biological water purification using bacteria living within the filter media. More precisely, biological filtration processes highly toxic chemical compounds dangerous to fish—which accumulate in the aquarium during the life activity of the fish themselves—into other, much less toxic compounds whose concentration in the aquarium can be significantly higher than the first ones and which can be easily assimilated by plants.
A good biofilter should have the following characteristics: a sufficiently large surface area of the filter medium (granulate, expanded clay, ceramic tubes, hollow plastic balls, etc. are used as fillers); intensive oxygenation of the incoming water (the filter design must provide aeration of the supplied water); sufficient filter performance (l/h). An averaged value can be cited: 3-4 aquarium water volumes per hour. As for the volume of the filter medium, it should be at least 1-2% of the aquarium volume.
Depending on the water flow rate, filters are divided into fast and slow. The source of water movement in slow filters is usually an airlift. The principle of its operation is as follows: • the air flow from the compressor in the form of small bubbles mixes with water; • the gas-air mixture rises in a narrow vertical channel (tube) above the water level in the aquarium. Thus, water is pumped through the tube, and the filter medium can be installed at the inlet or outlet of the tube. In fast filters, mechanical (most often electric centrifugal) water pumps serve as the source of water movement. They create a high water head, thereby ensuring a higher rate of water circulation through the filter medium.
In addition to filtration, there are several other water purification methods: chemical, flotation, adsorption filtration, ozonation, and ultraviolet irradiation. In these filters, ion-exchange resin, zeolite, coralite, peat chips, activated carbon, and other materials that actively change the chemical composition of water are used as filter media. Thus, for effective purification of water abundant in organic substances, it is advisable to use activated carbon as a filler, especially in aquariums with fry.
An adsorption filter (a filter with activated carbon or zeolite adsorbs dissolved organics and certain salts from water). Ozonizers also perform chemical filtration to some extent by oxidizing organics. Protein skimmers (remove organics and phosphates from the filter along with foam). Most filters can be used for this type of filtration. We recommend using either internal biochemical filters or external multi-chamber filters.
Filter systems. For fishkeeping, this is a relatively new concept: it denotes a whole range of elements making up the filtration system. Filter tanks are fixed to fittings so that they are submerged to a depth of only 10 cm. The upper tank protrudes above the water surface. The pump drives water through the filter mass, and the degree of filter contamination is very easy to determine.
Bioreactors, occupying a small space, ensure the decomposition of toxic nitrogen compounds that often and quickly form due to organic pollution of aquarium water (feces, food leftovers). The condition for this is the uniform operation of the unit and the associated spraying of the granulate filling the biofilter. If an osmolator is installed, it activates a dosing pump that adds water and thus always maintains a single water level in the reservoir. The main task of the osmolator is to maintain a constant water level and mineral content in this water.
Adsorption filtration. Adsorption is a process in which dissolved substances are absorbed by the surface layer of a liquid or solid body. The best known adsorption material used in fishkeeping is activated carbon. The importance of activated carbon is usually somewhat exaggerated. In any case, it does not remove nitrates or other nitrogen-containing ions from the water. All that carbon can do and does is remove relatively large organic molecules, amino acids, peptides, and certain gases from the water. Nevertheless, carbon allows combating active nitrogen indirectly, since ammonium and nitrite are precisely the breakdown products of those protein residues that carbon traps.
The use of carbon allows achieving very good water transparency in the aquarium; carbon is also used after fish treatment to remove drug residues from the water. The action of carbon is ensured by its exceptionally porous structure, i.e., large surface area. For more thorough water purification from dissolved organics or in “emergency” cases (when organics need to be removed quickly), filters with activated carbon granule fillers can be used. In addition to toxic compounds, it also absorbs acids (peat), various dyes and medications, and therefore filtration with carbon cannot be combined with the addition of acids or medications. Activated carbon should be of the BAU grade (Birch Activated Carbon). Other types of activated carbon may release harmful substances into the water.
With prolonged use of an adsorption filter, the granules become covered with bacteria, adsorption stops, and such a filter begins to work as a submersible biofilter. Dirt is the worst enemy of activated carbon and its porous surface. Therefore, the carbon filter in the general chain of filters must certainly occupy the last place, and all mechanical cleaning processes must be carried out beforehand. If the carbon is clogged and jammed with dirt particles, there is no point in it. Therefore, periodic inspection and processing of the granules are required. Many manufacturers offer so-called cartridge filters for their products: their outer shell consists of a mechanical filter material (perlon wool), while the inner cartridge, on the contrary, is empty and intended for filling with carbon, peat, or another similar filter mass.
Types of filtration. Chemical. Osmotic. Peat. Flotation.
Filters for water filtration. Filter – an essential part of the aquarium. 
Last changes and additions were made on November 10, 2012.
