Lymphocystis
Lymphocystis is a common viral disease of freshwater and saltwater fish.
Aquarists often come across this virus when their fish are stressed such as when put into a new environment and the virus is able to grow.
The fish starts growing small white pin-prick like growths on their fins or skin and this is often mistaken for Ich/Ick (Ichthyophthirius multifiliis) in the early stages. It soon clumps together to form a cauliflower-like growth on the skin, fins, and occasional gills.
There is no known treatment for this virus, though some aquarists recommend surgery to remove the affected area if it is very serious.
Eventually the growths inhibit the fish's ability to swim, breathe or eat, and secondary bacterial infections usually kills the fish.
Usually the best cure is to simply give the fish a stress free life, a weekly bacteria treatment and the virus will slowly subside and the fins will repair themselves. This can take many months.