@Silvia that’s what happened with mine - first it was the classic black spots at the edges of the fins, then i start seeing black slots in the middle of the fins. Then the black spots became no fin.
things really turned around when i used kanaplex and added salt. I started at 1 tbsp per 3 gallons and eventually got to 1 tbsp to 1 gallon. Then he really started to turn around.
i took him out of his planted tank and put him in a QT tank and put the heater up to 80F
Here is the thread