Actually, a UV sterilizer can be used in either a salt water reef aquarium or a salt water fish only aquarium.
What is the sterilizer? It is a container, usually in a near tube shape, in which an ultraviolet (UV) fluorescent lamp is contained. The lamp emits light at around 254 nm. The lamp is constructed of quartz glass to prevent the UV being absorbed as it would be if ordinary glass were used. The lamp is in a container of its own to shield it from the water, and then there is an outer container. The space between the containers is very small allowing full penetration of UV light. The water is passed between the outer and inner containers by a pump or power head.
The UV light is fully shielded from the aquarist as, of course, it could be damaging.
It is important that the unit used is recommended by the manufacturer for the size of aquarium it is to be fitted to, and very important that the pump used to circulate the water through the unit is of the correct size, again as recommended by the manufacturer. However, an aquarium cannot be ‘over UV’d’ as the UV light is completely contained.
What does a UV sterilizer in a reef tank do? The major job of the unit as far as aquarists are concerned is the destruction of minute free swimming parasites and other disease causing organisms. This occurs when these organisms pass with the water through the UV light. They are either killed outright, or are severely damaged and no longer a threat. (This is the reason it is important to have the correct sized pump. The organism must be in the UV area for a sufficient time.) The threat of disease is reduced, but not entirely removed. Even with a sterilizer in use, the aquarist must continue to be on guard for any sign of disease.
A further very important point is that the UV lamp has a limited life. In accordance with the manufacturer’s recommendations, the lamp must be renewed periodically. This keeps the UV unit at full efficiency. The renewal point will occur more quickly if the unit is run continuously, but continuous running is not necessary in normal circumstances. However, it is probable that most aquarists do run continuously. When feeding small live food that could be drawn into the UV unit turn the unit off.
The UV unit must be run continuously if a disease is suspected or confirmed. Many disease organisms have a free swimming stage and it is at this point that they are vulnerable.
It can be seen that the UV unit is of most potential benefit on a reef aquarium, or a fish only with live rock aquarium, as copper medication should not be used in these aquariums. A fish only aquarium can also benefit, but if a severe disease outbreak occurs medications can normally be used.
It is likely that a disease outbreak will not occur in an aquarium if all the proper procedures for introducing new corals and fish are used, and proper maintenance is adhered to. It is for the individual aquarist to decide if a UV unit is worthwhile.
Just remember that a UV unit is not an excuse for poor husbandry or cutting corners!