In an enclosed ecosystem such as an aquarium, it is imperative to the health of the fish that all environmental factors are kept in balance. Occasionally, the balance may become unstable or disrupted and this can cause an outbreak bacteria, parasites, viruses, fungus, or algae. When this occurs, conventional filters cannot remove these micro-organisms from the water and medication is usually the only recourse.

In some instances, medication may be an inappropriate treatment for a particular species and in a large aquarium, the amount of medication required to control a disease successfully, might have a negative effect on filter bacteria. Additionally, the quantity of medication required is often restrictive in financial terms. An alternative solution is the use of Ultraviolet sterilization.

Ultraviolet (UV) light is an effective method of eradicating water borne micro-organisms like parasites and viruses, as well as floating algae that can cloud the water. When micro-organisms are exposed to this light, the UV energy causes a change in the bonds of the DNA molecules and, with micro-organisms, this change can result in their destruction.

Ultraviolet light, naturally produced by the sun, is the name given to the light waves that fall between X-rays and visible light rays in the spectrum. UV light is invisible to the human eye, but consists of a high radiation energy level. However, by the time the UV light reaches the earth its intensity will have been reduced by such factors as atmospheric conditions, latitude, and time of the day. Commercially available lamps, similar to ordinary fluorescent lamps can also produce artificial Ultraviolet light. For combating disease, the most effective lamps have a wavelength of approximately 2537-Angstrom. (One Angstrom is approximately equal to one hundred-millionth of a centimetre).

Aquarium UV sterilizers work by forcing a thin sheet of water into close contact with a UV lamp. The lamp itself is sealed within a casing to ensure that no light can penetrate the aquarium itself. Water can neither store nor carry UV energy so only disease causing pathogens, harmful bacteria, mold spores, virus, protozoa, as well as algae floating in the water which is exposed to the UV energy within the UV unit are eliminated. Nitrifying bacteria, essential to the operation of biological filters, are not carried to the UV sterilizer as they live within the filter and thus will not be destroyed. Similarly, there is no risk to fish or plants.

The germicidal effectiveness or % kill of micro-organisms is controlled by the depth of penetration by UV light through the water and the time the water is exposed to the UV light. In short, the clearer the water, and the longer the contact time with the lamp, the more effective the UV sterilization. The ideal place to install a UV sterilizer is therefore after the mechanical filtration stage and before returning the water to the aquarium.

As with conventional filtration, maximum effectiveness is achieved only if all the water passes through the sterilizer at an optimum rate, nominally 1.5-3 times the tank volume per hour. Connecting a UV sterilizer to the output of a conventional filter and using the filter to pump water through the sterilizer will ensure the correct flow rate and that all water is processed.

The lamp's age also plays a major part in its effectiveness. As with conventional fluorescent tubes, UV lamps degrade with age. Most UV lamps will need to be replaced every 6 months. Multiple start and stop cycles will shorten the life and effectiveness of the lamp and in addition, stop cycles will again allow the multiplication of dangerous micro-organisms so the sterilizer should be run continuously.

UV light (radiation) does not affect taste, odour nor produce any notable chemical changes in the water, making it an ideal weapon in disease prevention and a useful tool in the goal of creating the perfect environment for your fish.