Fin nipping or fin rot?

Hello, I have a 55 gallon tank with a 20 gallon sump and 4 medium discus currently in it. Water parameters are perfect and nitrates are no higher than 5ppm currently. They are all acting fine, however at least two of them are picking on my white one pretty often. I’m beginning to notice something that looks like either nipping Or fin rot on him and I need help figuring it out. Picture is attached. Do I need to add a couple fish ASAP to get the aggression down?
Attachment 115165

This is very common. The fin should recover in a few days.

Looks like the damage is healing on its and closing up. It doesn't look like fin rot to me at this stage.

al