How Do UV Sterilizers Work?

There are all sorts of equipment that could be used to assist maintain the aquarium, each designed for a specific purpose. Some of this equipment is essential and there is no debate about its need, for example heaters. Other equipment could be cause for debate.

A piece of equipment that does cause some debate about its use and necessity is the UV sterilizer. UV stands for ultraviolet. Some aquarists use it, some do not. It is often seen in the LFS (local fish shop) as they try and combat the danger of disease.

UV light is dangerous to the human eye and is also dangerous to certain tiny life forms that could find their way into the marine aquarium. With a UV sterilizer attached, the aquarist hopes that any disease organism will be prevented from becoming a serious nuisance. The UV sterilizer is an aid to disease prevention, not a total answer.

The sterilizer is usually an elongated tube shape. Basically there is an outer casing, an inner glass casing, then the bulb. Ordinary glass cannot be used as it absorbs nearly all the UV radiation, therefore quartz glass is used which is of the type that prevents UV radiation below 220nm (nanometers) from passing through. (This is because of the problem of ozone generation, ozone is produced below 200nm.) The wavelength produced is stated to be 254nm. UV penetrates seawater to about ¾” (circa 2cm); therefore the seawater flow past the UV bulb is restricted to this depth.

Seawater flow rate is important. Too fast a rate and organisms will not be exposed for long enough. Therefore it is very important to check the manufacturer’s recommendations for the flow rate to ensure a suitable pump is connected. In addition, the bulb has a finite life and again the manufacturer should give information. Once ‘time is up’ the bulb must be changed.

Proper exposure to UV will kill or damage free floating algae, bacteria, protozoans etc. Pond keepers make use of UV as well as marine hobbyists.

A downside of using UV with a marine aquarium is that anything that is small and is irradiated will be affected. There isn’t any differentiation between good and bad. The bio-filter is an absolute requirement for the health of the aquarium and worry might be caused as the bio-filtration is performed by bacteria. However, these bacteria are not harmed as they are resident within one place, either live rock¸ a canister filter or similar. They are not exposed to radiation. If there is plankton in the aquarium then these could be exposed. However the plankton count in an aquarium is usually either very low or non-existent. If there is a presence they are likely to be damaged by impellors.

The major use for UV with a marine aquarium is assisting with the prevention of disease organisms and assisting with the treatment of the same if disease appears.

Major problems that could strike are caused by single celled parasites, which attach to fish and, if untreated, are likely to reach such numbers that fish death occurs. Fortunately there is a chink in the parasite’s armour, and this is that they have a free swimming stage, when they are known as tomites. It is when they are in this stage that exposure to UV is effective.

The parasites attach to the fish and after a fairly short period drop off. They fall to the bottom and divide into more parasites. They then return to the seawater column looking for fish to infect. Then the cycle begins again. When they are free swimming the UV can severely damage them. As I understand it their DNA ‘memory’ is disrupted, which means that when they reach the divisive stage, put simply they can’t – so no more parasite production.

It sounds like the complete answer to some disease problems but unfortunately it isn’t. For the UV to inflict damage the parasite has obviously to be exposed to the light. Can that be guaranteed in an aquarium? Unfortunately not, just consider the reef or aquascape with all the caves, nooks and crannies, also the sand bed. Then there are the areas in the aquarium usually low down or in corners where seawater flow is low. The seawater flow close to the intake of the UV sterilizer could cause parasites to get sucked in, but there are more likely than not to be those that don’t in other aquarium areas. One or two parasites could restart the whole process.

A UV sterilizer, properly set up, is a good aid for disease prevention and treatment. It is not an answer in itself and cannot be relied upon as such. The main defences must always be in place: careful fish selection, careful introduction, appropriate feeding and high quality overall aquarium husbandry.

(Reference: Baensch Marine Atlas. Helmut Debelius & Hans A. Baensch)


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