The use of ultraviolet light on aquariums has a long history.

Back in the day—before reef aquariums came to North America—the aquarium trade introduced “ultraviolet sterilizers” to the hobby. UV was the hottest new water disinfection technology for drinking water. UV light sterilizes water without the use of chemicals like chlorine bleach and biocides. When micro-organisms such as algae, fungi, bacteria and parasites are exposed to UV radiation, it damages their DNA and they die.

So UV sterilizers were being used to purify drinking water. Commercial aquaculture was also using UV to reduce diseases in high-density grow-out operations. Aquarium equipment manufacturers thought UV sterilization would be a good idea for home aquarium use. It was believed that UV would make disease and algae problems a thing of the past. But that’s not what happened.



Research to the Rescue!

A lot of aquarists, fish shops and wholesale fish distributors installed UV sterilizers on their tanks and fish holding systems thinking their disease problems were solved. The problem was no one saw a reduction in disease outbreaks. A series of independent research projects were conducted by universities to see how well UV sterilizers work in aquariums, zoos and aquaculture facilities. Here is a summary of what was found out about UV sterilizers and aquariums.

  • Aquarium water is packed with micro-organisms—bacteria, plankton, algae, fungi, parasites and other microscopic life – hundreds or thousands of times what is found in nature
  • Aquarium water is full of microscopic non-living debris—even if the water looks clear
  • UV sterilizers don’t sterilize aquarium water—they can only reduce the population density
  • Some micro-organisms are harder to kill than others
  • Aquarium-style UV units are not powerful enough to significantly reduce the population of disease-causing organisms like Ich and bacteria.

For organisms to be killed by UV, they must be exposed to the radiation at a certain strength for a certain amount of time. Because aquarium water contains so much suspended matter, it is easy for the organisms to “hide” behind other particles and organisms as they flow past the UV light. This reduces exposure to the UV radiation and limits its effectiveness. At the same time these “bugs” are reproducing in the aquarium.

Small aquarium UVs can help control but not eliminate parasites, bacteria, etc. Overall the research demonstrated that UV somewhat reduced the population of suspended organisms. But there was no sterilization, no complete elimination of disease organisms. If you want to sterilize your aquarium water, you’ll have to buy a massive commercial UV unit costing thousands of dollars.



Why Use a UV on a Reef Aquarium?

Today UV “clarifiers” are sold primarily for green water algae control in ponds. Aquarium UVs will clear up stubborn algae blooms in freshwater, marine and reef aquariums in a matter of days.

UV units have no effect on algae that is already growing on the glass or live rock. Some aquarists worry that UV will eliminate the natural plankton population in their tanks. Based on the wealth of research it is safe to say that running an aquarium-style UV on a reef tank won’t have much of an effect on plankton.

You may wonder how this can be true since a UV can clear up green water. The fact is, your crystal-clear water is full of millions of algae cells, bacteria, crustaceans and other microscopic life. Even though you can’t see them, your corals are feeding on this “soup” of plankton. A reef tank is the ideal breeding ground for marine plankton and it is impossible to eliminate them with an aquarium UV sterilizer. The relatively small portion of the tank’s water that passes through the UV unit will have the most effect on the dense concentration of rapidly reproducing algae cells.

Some reef-keepers never use UV while others say their aquariums never looked clearer since adding a UV. The bottom line is it is up to you to decide if a UV sterilizer is a useful tool for your situation.