Originally Posted by Snipe
If it's truly a fungus, that happens about 99% of the time due to a couple of things.. Mishandling or water quality issues. The common element in many cases (not all) is stress, which directly affects the immune system.
Rapid changes in weather (temperature swings), heavy fishing pressure, massive inflow, sudden turbidity..

Well said and very common ! Especially a stress issue which can be water quality , poor condition , overcrowding , poor nutrition , spawning and others.