Discus fish are prone to stress, diseases, and illnesses more than any other freshwater aquarium species. Viral infections, bacterial infections, parasitic diseases, and fungal infections are very common in Discus.
Viral Diseases
The Discus Plague, is the worst disease/condition that discus fish can suffer from. There is no known cure for it, and mortality rate is very high. It does not kill fish directly, but through weakening the immune system and getting the fish system ready for other (secondary) diseases.
Parasitic Infections
Parasites, including Helminths and protozoa kill discus fish more than other micro-organisms! Parasitic infections especially internal Hexamita and intestinal worms are some of the hardest discus fish diseases to treat. Infestation of single-celled protozoa also is not easy to cure. The cross contamination is actually the infestation of single-celled parasites such as Costia, Chilodonella, Trichodina, Ich, etc.
Bacterial Infections
Discus suffer from different types of Bacterial diseases and conditions such as Columnaris, Pop-eye, Dropsy, etc. Small wounds and scratches on skin that produced by parasites also get infected by secondary bacterial infections very quickly.
Fungal Illnesses
Fungal illnesses are not so common in discus fish unless the quality of the water is very poor and fish are suffering from weakened immune system, but when occurs is really hard and also time-consuming to treat.