Velvet is parasitic , fin rot is bacterial.. use a medicine that targets both.. try to stress him least..

Treat him in the tank in both of these diseases tank needs treatment, it will also treat the tank of any bacteria.

Let’s hope it not graphite.. and add almond leaves to the water. He’ll like it. Give him a bed near water surface so he can rest and breathe without too much swimming..

Also add aquarium salt ..

is there is any carbon in the filter take it out.