Devices for water purification in the aquarium – filters.
To be continued…
Chemical filtration. This method is used least frequently. The task of chemical filtration is to trap toxic molecules contained in the water. But no matter how good the absorbing filter media are, they only “bind” these substances. The aquarist has to remove them by cleaning the filter and finally taking out the masses saturated with toxic substances. It is mainly useful when there are short-term problems. For active aquariums, a special type of chemical water purification associated with filtering it through ion-exchange resins or certain other substances is of particular interest. Nitrate removal using an ion exchanger is another option, but it requires increasing water changes.
It is a specific type, as it removes certain compounds from the water for which the applied reagent is intended (ammonia, heavy metals). In this case, synthetic, ion-exchange resins or natural minerals from the zeolite group most often serve as the filtering material. Zeolites and resins absorb ammonium, nitrates, phosphates, etc., and release harmless sodium, chlorine, and sulfate ions instead. The use of high-moor peat as a filter medium can also be considered chemical filtration. Peat slightly acidifies the water and introduces various biologically active substances into it.
Peat filters. Peat filtration is a phrase rooted in aquaristics, but strictly speaking, it is not entirely correct. After all, a filter, as we know, should trap something, and if it is filled with peat, it, on the contrary, gives something to the aquarium water! Many tropical waters have greater or lesser acidity. We are talking here about humic acids, which are released by wood and foliage. These organic acids can be introduced into the aquarium by directly adding humic extracts or by passing water through peat so that it absorbs the substances contained in it. A wide variety of peat grades are available for sale. Peat is a natural product, and no matter what kind it is – loose or dense, after a few weeks it washes out and no longer gives anything, it needs to be replaced.
Water that is enriched with humic substances by passing through peat acquires a special color, from brownish to amber. But its shade does not tell anything about the presence of acids and their features. Acid lowers the pH value to the neutral point = 7.0, and therefore it makes sense, when using peat, to establish constant control over pH indicators. Peat also serves to reduce the carbonate hardness inherent in most types of water. In addition, acidic water keeps the number of bacteria within certain limits, which benefits the skin of many fish species: bite wounds do not become inflamed under the influence of bacteria. On the other hand, a great many species of ornamental fish originating from particularly clean and acidic waters are known: with the help of peat, they are placed, at least approximately, in conditions close to natural.
Flotation – the removal of unwanted impurities from water along with the foam formed. The essence of the method is based on the ability of many substances to adsorb on the surface of 
gas bubbles passing through the liquid. Gas bubbles rise to the surface of the liquid, forming foam in which unwanted impurities are concentrated. The foam along with impurities is removed, and thus water purification takes place. The principle of its operation is as follows. In order to prevent excrement and other fish secretions in the form of mucus and high-molecular compounds from entering the mechanical filter, where their primary decomposition usually occurs, causing poisoning of the entire aquarium with organic substances, so-called skimmer chambers or protein skimmers are used.
In these devices, which are tall cylinders filled with water, tiny air bubbles rise, often with the addition of ozone. These bubbles adsorb particles of organic suspension, mucus, and high-molecular secretion components on their surface and remove them outside the aquarium in the form of foam into separate containers – foam collectors. Periodically, the collected foam is poured out, and the foam collectors, having been rinsed beforehand, are returned to their place. The purified water is fed into the aquarium or other purification devices. Flotation purification is best carried out before biological filtration.
There are two basic designs of protein skimmers – direct-flow and counter-current. In the first case, the flow of treated water moves in the same direction as the rising bubbles. In the second case, the treated water moves towards the bubble flow. Since the second system is recognized as much more efficient due to the longer retention time of water in contact with bubbles (this parameter is called “retention time” in foreign literature), direct-flow skimmers have currently lost their relevance. The protein skimming column works best in salt water. Freshwater models are ineffective and bring no real benefit.
Diatomaceous filters are used to clean aquarium water from particularly fine particles. Diatomaceous earth is used as the filter medium for diatomaceous filters. This very fine yellowish-cream earthy mass consists of the siliceous shells of fossil diatoms (microscopic algae). The filter housing (glass vessel) is equipped with a powerful pump – the filter resistance caused by the low permeability of diatomaceous earth is higher here than in other filters. Anyone who wants to process the water in their aquarium particularly thoroughly (achieving special transparency or reducing the number of parasites and bacteria, or completely destroying them for the purpose of breeding fish) purposefully connects this filter for many hours.
At the same time, this is purely mechanical filtration that does not affect water parameters or chemical additives (medicaments). Unfortunately, because they trap the smallest particles, they quickly clog and require replacement of the filter substrate. In addition, the necessity of using such systems for ordinary aquariums is questionable, as fish in water devoid of almost all life begin to lose mucus from their skin, which negatively affects their health.
Osmotic water filtration (reverse osmosis). If hard water flows from your tap, and you want to keep and, moreover, breed discus fish, which love acidic and soft 
water, then you will have to think about how to prepare the appropriate water one way or another. Osmotic filtration is based on the property of water to penetrate through a porous membrane impermeable to dissolved substances. If increased pressure is artificially maintained on one side of the membrane, the equilibrium will shift. The operation of a reverse osmosis filter is based on this principle.
Metaphorically speaking, water is pushed through the membrane, while mineral salts remain behind. The membrane divides the flow from the regular water supply into two parts: one, representing purified water, is used for domestic needs, and the second, enriched with impurities reflected by the membrane, is drained into the sewer. Unlike deionization, osmotic filtration is a mechanical process that allows filtering other substances as well – for example, organic molecules and even bacteria.
Water deionization (deionization). Such filters use chemically active filter media. They replace ions dissolved in water with others. One medium filters positively charged particles – cations, replacing them with a hydrogen ion H+ (cation resin), the second replaces anions with negatively charged OH- ions. The combination of the two filtration stages forms a water molecule. Such filtration occurs as long as there is a sufficient number of ions in the medium for exchange. After exhaustion, the resin can be recharged using alkali and acid. However, it is better not to do this yourself. Some resins, especially those recommended as a filler for a regular aquarium filter, replace cations with a positive sodium ion. Such resin can be regenerated in a strong salt solution.
However, an increased concentration of sodium ions can be harmful to some delicate fish and not very good for plants, blocking their absorption of certain trace elements. It is better to use two media – for cation and anion filtration. Sometimes both media are mixed together. Such a filter will be much harder to recharge because it will be necessary to separate one resin from the other before regeneration. In principle, such filters allow obtaining cleaner water than osmotic filters. For example, they filter out silicates well, which is necessary in a coral reef aquarium. Service life and performance depend on many factors, especially the mineral content in the source water. With very hard water, the medium may “ask for regeneration” after 100-200 liters. To increase the service life of such a filter, it can be placed after the osmotic filter.
Internal aquarium water filters. Operating principle of the internal filter.
Types of filtration. Filtration systems. Adsorption filtration. 
