I have one similar to that with black smudges. When he fully colored up, his scales were lined in black on the front half of his body. All of mine that had bad ammonia wound up with bright red scarring on the top of their heads and on their gills. I'm seeing a little on his head so it looks like you got to him before he burned badly. He's a pretty boy! And yeah, i've been there on the chain store rescue before. Clean, warm water does wonders! Someone else may be able to tell you if they see fin rot as I've never dealt with that.