UV Sterilizers and Freshwater Discus Aquariums

Q. I have a 90-gallon freshwater aquarium that is bare-bottomed and houses discus fish. The aquarium seems to be doing well with the following parameters:

• 7.0 pH
• 86 degree Fahrenheit water temperature
• 11 adult discus fish and cory catfish

My friend recommends a UV sterilization system for freshwater aquariums housing discus fish. Is this necessary? I know they are popular in saltwater aquariums, but I have not heard of them being used in freshwater aquariums.
Chris Monroe II
Delaware

A. I kept discus fish for a number of years and never implemented a UV sterilizer on any of my discus freshwater aquariums. In fact, I don’t think I ever used a UV sterilizer on any freshwater aquarium. As you mentioned, UV sterilizers are very popular in saltwater aquariums, and I have always used them on that front. There would be advantages to a UV sterilizer on any aquarium, though, whether or not it is necessary is questionable.

Discus fish are susceptible to several parasites in the freshwater aquarium. While most external parasites can attack discus, the parasite most associated with them is the Heximita protozoan. This is an internal infection that many discus fish acquire when their immune systems are suppressed during transport. Water temperatures below 80 degrees Fahrenheit have also been linked to outbreaks of this internal parasite in discus fish. If the infected discus fish is not treated, the parasite is deadly.

The other illness commonly associated with discus fish is the infamous discus plague. This disease (thought to be a virus) is in many cases fatal and known to wipe out entire aquariums full of discus fish. However, I am not convinced that a UV sterilizer would be effective in keeping your discus fish healthy in either case. The Heximita infection is internal, so I can’t see the protozoan passing over the UV sterilizer where it would be killed. The discus plague is thought to be viral, and I am not sure the UV sterilizer could do much for that either.

The most important factors in keeping discus fish healthy are aquarium water changes and being very selective about where you acquire your discus. I obtained the best results when I found one reputable discus breeder that had healthy discus fish and stuck with those. When I tried to mix and match discus fish from other areas, I often ran into problems. Aquarium water changes, as any discus keeper knows, are very important, not just because these amazing fish require pristine water, but also to remove the growth-inhibiting hormone released by adult discus.

One thing to be very mindful of are wild-caught green Heckel discus. Oftentimes, if these discus fish were wild-caught as advertised, they can harbor an immense amount of parasites. I know several discus keepers that lost stock due to the introduction of wild-caught discus fish. Also, discus imported from Asia are thought to be responsible for the introduction of the discus plague to the United States. Luckily, many American breeders offer the same color varieties and options in discus fish that Asian breeders offer.

Implementation of a UV sterilizer on your freshwater aquarium would be a waste of money. I would continue to keep good water quality in your freshwater aquarium, feed premium foods to your discus fish, and monitor where any new discus fish are coming from. The only real advantage you might find with a UV sterilizer would be a reduction in algae growing in the freshwater aquarium.