Virus infection. Large white lumpy growths become evident on the fins of the fish.
Usually not fatal. Left alone, the fish often recovers spontaneously.
DNA and RNA diagnostic references
The Complete DNA Sequence of Lymphocytis Disease Virus
From Virology Volume 230 Issue 2. 14 April 1997 pages 207-216
Christian A.Tidona and Gholamreza Darai.