Could you please give a little more information. What are your tank parameters: ammonia, nitrite, and nitrates. What is your fishload, tanksize and water change schedule? Have your fish been acting unusual: clamped fins, listless on the bottom, eating less, shimmy (acting like it is swimming but not moving)? Based on your description, I am thinking clout which is bacterial, usually caused by weakened immunity due to some form of stress and maybe your tank water test results/tank parameters would confirm this.