UV Sterilization and the Reef Aquarium

Ultraviolet (UV) light is invisible to our eyes, with it’s UV energy broken down into three bands: UV-A, UV-B and UV-C. Black lights typically use UV-A. Sunburn is caused by exposure to the UV-B band. UV-C radiation is “germicidal”, meaning it will kill microorganisms by damaging their DNA. Germicidal UV radiation is created with specialized mercury vapor lamps that produce light in the 100-280 nm range (UV-C). With this knowledge ultraviolet technology was first used to disinfect drinking water in Europe. The UV units were very large, with banks of UV bulbs contained in stainless steel cylinders. The technology was eventually adopted by the aquaculture industry. The idea was to pump culture water through the UV unit to kill pathogenic organisms like bacteria, fungi and parasites. UV sterilization adds no chemicals to the reef aquarium and has no affect on the aquatic life. The light is contained inside the UV chamber. This technology was embraced by the aquaculture industry because they were not permitted to treat the fish with antibiotics and other chemicals. UV reduced disease problems without the use of medications. As the technology matured and became less expensive, small UV units were produced for aquariums.

UV Sterilizers Misconceptions

UV sterilization was not popular with early reef aquarists. Smithsonian captive reef pioneer Dr. Walter Adey used special water pumps designed to move water but not harm plankton in the reef tank’s water. The idea was plankton essential to the natural food web would be wiped out by impeller-driven pumps, disrupting the tank’s ecosystem. Since UV sterilizers were intended to kill anything in the light’s path, many aquarists thought UV would have a negative impact on their reef. The idea that a hobbyist-grade UV sterilizer could “sterilize” an aquarium was disproven by research conducted at the University of Georgia.

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Diagram of how a UV Sterilizer plumbed into the aquarium
Diagram: Directional flow of water with free-floating cells and filamentous fragments.

In aquarium studies researchers discovered that UVs do reduce bacteria and parasites but never eliminate them from an aquarium. That’s because UV only kills what passes by the light. Microorganisms living in the gravel and rock are safe. Plus, the small hobbyist-sized UV units are too small to handle the high water flow required to completely “nuke” the tank’s water every hour. Interestingly, there was a phase in the hobby where aquarists thought SPS corals never fed on plankton and only required to light to survive. But all that’s changed. Today we know corals use light energy and consume planktonic foods. So where does UV sterilization fit into reef-keeping?

Benefits of Ultraviolet Sterilization

Benefits of UV Sterilzation

Through years of practical experimentation by reef aquarists we’ve learned that nothing “bad” happens when there is UV sterilization run on a reef aquarium. The biological balance isn’t disrupted. Corals don’t starve. And most importantly, genetically modified UV sea monsters don’t climb out of the tank. What we do see is less algae problems. While UV won’t kill algae on the rock or glass, it will kill free-floating cells and filamentous fragments that pass through the UV unit. This ultimately reduces the algae population and brings algae under control. Although your aquarium water appears clear, there are millions of bacteria and other microbes floating around. The bacteria load in aquarium water is enormous compared to natural waters. A UV will dramatically reduce this bio-load, causing the aquarium water to sparkle with clarity. If some of these microorganisms happen to be pathogenic, you’ve also reduced the chances of disease problems with fish and corals.

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Things to Keep in Mind

There are many models to choose from ranging from drop-in UVs for nano and all-in-one aquariums to large in-line units that require hard-pipe connections. Select the UV that is rated for your size tank. If possible, position the UV in a flow of clean water free of particulates. Microbes are shielded from the light by floating particles, reducing efficiency. Even though an old bulb still lights up, it’s effectiveness declines after about one year. So, change the UV-C bulb once a year to reduce your tank’s chance of a algal, parasitic and bacterial outbreak.

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