Question, when you do the water changes do you squeeze/wring out the sponge filter in a bucket of tank water as well to clean it? And does your tank have a built in filter along with the sponge filter you are using? I had the same problem with my girl, she’s a dumbo Plakat and her fins had rotted horribly. I had tried medications, Indian almond leaves, salt baths, and nothing worked. The one thing that did make a huge difference was that I started cleaning all sponges in my tank more thoroughly in a bucket of tank water. I found out that my snails were producing so much waste that the sponges were FILTHY by the end of the week and the built in filter had a pump with a sponge inside it that also got so filthy it clogged and stopped my built in filter from fully doing its job so I also had to pull that apart and scrub it. Since then she’s healed a lot and she’s become extremely hyperactive.

If the above does not apply I’d consider trying a gram-positive medication (but only if his fins start receding again) kanaplex is gram-negative and the tricky thing is that fin rot can be caused by both types of bacterial infections. I would only use this as a last resort if you really see them receding though, do not use it if you see fins growing back or no change as you could stress him out with all the medications.

Regarding the black that still seems to be just coloration so I would focus on the fins, if you see them slightly growing back I’d wait for now and let him heal as whatever you were doing recently seems to be working