Is a UV Sterilizer Bad for a Reef Tank?

These filtration systems work by drawing up a small amount of tank water at a time, usually through a powerhead motor, and passing it over a UV bulb. The procedure kills fungi, algae, bacteria, parasites and sometimes viruses, depending on the strength of the bulb and the amount of time the water spends under UV light. Some UV systems claim to kill as much as 99 percent of the organisms that pass through them. The system can help hobbyists prevent disease and parasites in their tanks and get a handle on algae blooms.

In addition to killing harmful organisms, UV light can break down essential chemical compounds in the additive products you use in your tank. If you do get a UV system, you will need to turn it off for eight to 24 hours any time you put an additive or medicine in your tank. Allowing essential chemicals to go through this filtration system often leaves them weakened or useless.

The effects of UV filtration on beneficial bacteria and organisms are largely determined by how the system is implemented. Never place a UV light directly over the tank, as this will kill beneficial bacteria and may harm the species kept in your tank. When the UV system is kept on a separate line, it will not affect the beneficial bacteria from a biological filter. UV light will kill amphipods and copepods, so if you are trying to foster these organisms, UV sterilization may not be right for you. However, adding a refugium to the tank on a separate line for these organisms can allow you to have the best of both worlds. A UV filter will not affect bacteria that are on surfaces or in the substrate, which can create a refuge for both beneficial nitrifying bacteria and harmful disease-causing organisms.

There may also be some risk to your tank if the UV system is not properly maintained. The rubber O-ring that seals the UV lamp in a quartz tube may grow weak or brittle over time. This allows humidity to reach the electrical areas of the system, leading to shorts and the production of potentially toxic chemicals that will flow from the system into your tank. Provided you check the O-ring for damage every time you clean the system, this type of breakdown should not be an issue.

In the end, deciding on a UV sterilizer is a personal choice. It can be a good tool against tank-wide illnesses and parasites, and may help you get a handle on an algae problem, but it is far from a complete solution. Any bacteria, parasites or algae that are already on surfaces, in the substrate or in the organisms in your tank will be unaffected, while useful amphipods and copepods may be killed. UV sterilization could make a useful tool for your collection, but it is up to you to weigh the costs.