Help my poor Goldie

[ShawnInfirmity]

Now my fish seems to be developing black on his fins and under his neck. He is swimming fine and acting like his old self though. Could this be from the treatment or is this an entirely separate issue?!

The black coming afterwards could be a sign of healing much like what happens to them after they’ve been exposed to a high level of ammonia. If you look up “ammonia burns in goldfish” it looks a lot like what your photo shows.