Ultraviolet lamps – what they are needed for in aquarium keeping.
Working alongside the filtration system, a UV sterilizer reduces the number of factors affecting the fish’s immune system. Suppose you slightly overfeed the fish, and after a short time, if the biofilter’s capacity is low, the mass of free organics in the water—leftovers of uneaten food and metabolic products of fish and other aquatic organisms—increases sharply. Bacteria floating freely in the water column, including pathogenic ones posing a danger to fish, begin to decompose the organics right in the water and actively multiply.
Very soon, the number of pathogenic bacteria reaches a certain critical point, after which the fish’s immune system (each species has its own critical point) can no longer defend the body against them and the protozoa multiplying in their wake. A UV sterilizer effectively fights floating algae like Chlorella and Euglena, i.e., water bloom. However, it cannot have any effect on attached algae in the aquarium.
Remember, bacteria are consumed by protozoan microorganisms, many of which are fish parasites. A chain reaction begins. Thus, the body’s resistance to pathogenic flora and fauna decreases, and fish begin to get sick, usually with several diseases at once, such as mixed infections. If you fail to understand the situation in time and make the right decision, a die-off will begin. In such a situation, the benefits of UV water sterilization cannot be overestimated. If this device is properly selected and installed, you eliminate one of the most severe factors affecting your fish’s organisms: the uncontrolled reproduction of pathogenic microorganisms in the water. Therefore, even if you make some mistakes in feeding, filter washing, substrate and decoration cleaning, and so on, in most cases you will be insured against this trouble.
Ultraviolet (UV) rays can cause fatal damage to the cell membranes of bacteria and viruses. Through photolytic reactions, ultraviolet light breaks the bonds of organic molecules, suppresses their activity, destroys DNA and RNA, damages cell membranes, prevents cell division, and denatures proteins. For most microorganisms, including pathogens, such damage becomes fatal and irreversible even at a relatively low irradiation dose. Like a high-precision weapon, ultraviolet targets living cells specifically without affecting the chemical composition of the environment, which occurs with chemical disinfectants. This latter property exceptionally distinguishes it from all chemical disinfection methods.
The ultraviolet sterilizer works in harmony with other existing filters. Water treatment with ultraviolet rays allows for its disinfection: various unwanted microorganisms that develop in large numbers when keeping a large number of fish in a closed aquarium system are destroyed. By this method, water can be freed from bacteria, viruses, fungal spores, and some protozoa. Ultraviolet creates a more favorable microclimate in the tank, reduces the risk of epizootics, decreases the pathogenic load on the inhabitants of the body of water, ensures water transparency by destroying bacterial turbidity, and prevents the mass development of microflora that causes, in particular, water blooming. UV irradiation has no therapeutic effect once a disease has broken out.
A UV sterilizer is a device used to disinfect water from bacteria, fungi, viruses, algae, and protozoan microorganisms, many of which are pathogenic 
and pose a direct threat to the health and life of aquatic inhabitants. By treating water with hard ultraviolet radiation with a wavelength of 250 nm, it allows for controlling the number of causative agents of many diseases in aquarium and pond fish. After mechanical and biological filtration systems, a UV sterilizer is the second most important equipment that can significantly improve water quality in the aquarium.
The working principle is as follows: water from the aquarium, under pressure created by a pump, passes through a filter and is fed into the sterilizer, which is usually located outside the aquarium (in a cabinet, on a shelf above or below the aquarium). Inside the sterilizer, the water is treated with an ultraviolet lamp and, exiting from the side opposite to the water intake, flows back into the aquarium. This cycle happens continuously. The longer the water is exposed to ultraviolet light, the greater the effectiveness of the irradiation. The irradiation time is strongly influenced by the current in your aquarium, as well as the length of the lamp. The slower the current and the longer the lamp, the greater the dose of ultraviolet radiation the water will receive. Another important factor is the “turnover” time, i.e., the time it takes for the entire volume of water to pass through the sterilizer. Since water from the sterilizer constantly mixes with water from the aquarium, it is impossible to sterilize all the water at once.
Design. Inside the housing of germicidal lamps (referring to classic in-line flow models), there are two tubes—an inner one made of quartz glass and an outer one made of PVC, hermetically connected by rubber and special sealants at the ends. From the outer tube on opposite sides near the ends, two fittings are welded, extending beyond the decorative protective casing of the device, to which hoses for water intake and return are connected. The germicidal lamp itself is located inside the quartz glass bulb. Its rays pass unobstructed through the quartz glass and destroy all living things in the water passing between the inner and outer bulbs.
Three parameters of the sterilizer determine the ultraviolet dosage it provides.
1. Power. Everything is clear here. The higher it is, the stronger the dosage, and the more positive the effect of such a device.
2. Working gap size, i.e., the thickness of the treated water layer. Water absorbs ultraviolet very quickly. Theoretically, in water of medium transparency, which aquarium water belongs to, complete absorption of ultraviolet rays occurs in a layer 40-60 mm thick. It can be considered that such a layer utilizes 100% of the lamp’s radiation, but the treatment quality is relatively low because only a small layer of the flow closest to the lamp receives a sufficient dose of irradiation. Accordingly, with a large working gap between the two bulbs of the device, the water needs to be passed through the UV sterilizer multiple times for complete treatment. Reducing the working gap noticeably increases the efficiency of water treatment.
3. Flow rate (capacity). This is more complex. On the one hand, the higher the capacity of the sterilizer, i.e., the water current passing through it, the less time the water spends under the influence of ultraviolet light, the smaller the dose received, and consequently the expected effect. On the other hand, to improve the quality of water disinfection, it is desirable that all the water in the aquarium be treated several times a day, and here is why. If, for example, the average division period of a bacterium is taken as 12 hours, then, as many authors write in their books, two water volumes must be passed through the UV sterilizer per day.
Based on all this and knowing the power of the UV lamp, its length, and the thickness of the working water layer inside, one can calculate the optimal pump performance for an aquarium of any volume. A very important criterion is the time during which water remains in the irradiation zone when passing through the sterilizer. That is, the longer the UV lamp along which the water flows while in the irradiated zone, the more the water treatment efficiency increases.
Safety rules. In addition to standard electrical safety rules, the following rules should be observed:
1. Never look at a lamp emitting UV radiation. Do not turn on the UV lamp without its protective housing under any circumstances. You can get a corneal burn, even go blind, and suffer severe skin burns even with short-term exposure. Regular dark glasses do NOT protect against such radiation.
2. Turn off the UV sterilizer when using anti- and probiotics, as well as certain other medications, particularly aniline dyes (malachite green, methylene blue). Antibiotics and many organic dyes decompose in it, cease to have a therapeutic effect, and their degradation products can poison the fish. According to some data, pauses are also desirable when applying plant fertilizers, since some chemical compounds (including iron-containing ones) under the influence of photolysis transform into a form unacceptable to flora or even toxic to fish and invertebrates. If you are just starting a new aquarium and have poured an industrial bacterial culture into it, you should not turn on UV sterilization for the first 2-3 days after that.
3. It is best not to use UV sterilization when adding fertilizers for aquarium plants to the water. Ultraviolet treatment of trace-element metal salts, which are necessarily part of fertilizers, can convert them into a form toxic to fish.
4. If the lamp is submerged in water, use grounding and a residual current device (RCD) that disconnects power in case of a ground fault. Such a safety device is recommended for any electrical appliances used in an aquarium.
UV sterilizer design. Factors affecting sterilizer performance.
Additional equipment for aquarium care. 
Last changes and additions made on May 24, 2012.
