Mastering Freshwater Aquarium Ecosystems
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Fish Health (Cont.)

Many diseases can live at low levels within the aquarium ecosystem without becoming a life threatening infection until water quality deteriorates and becomes a stressor on the fish’s immune system. Many adult fish can tolerate low level of parasites, but that same low level can cause mass mortality in juveniles and fry.


New viruses are being discovered in ornamental fish on a regular basis since the mid 1990’s. Viruses have become common in many species of fish in the ornamental fish trade. They are often species specific, affecting only one species, but there are exceptions. Many viruses have high mortality rate, and any survivors then become carriers that can go on to infect naive fish. Ornamental fish viruses have been responsible for millions of dollars in hatchery loses. Many hobbyist have also experienced catastrophic losses to a virus.

Unfortunately viral diseases are not curable, and need to be isolated in a strict quarantine system. If you detect a virus in new fish in your quarantine system, all new fish should be euthanized with Tricaine methanesulfonate (MS-222) as soon as detected. Water that had fish in it with a deadly virus should be considered a biohazard to any fish of the same species if the disease is species specific.

Discus Plague Virus

A common discus (Symphysodon sp.) virus often referred to as discus plague has been in the hobby since at least the 1980’s if not longer. Discus plague is believed to be caused by a virus, as it has been found in many discus that have shown the classic symptoms (per conversation with Tom Waltzek, DVM, PhD, University of Florida). Discus that have contracted this virus have very obvious symptoms which include, dark background color, clamped fins, heavy body and fin mucus, and hiding in a dark corner of the aquarium. Discus plague has a high mortality rate, often 50 to 90%. Discus that survive the the initial symptoms, which can last up to three weeks, will become carriers of the virus.

Discus Plague

Discus displaying classic symptoms of being infected with the discus plague virus.

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