One small patch on Bellus Angel tail fin

[nivenethan]

I have a Bellus in QT treating with copper that has a small irregular shaped offwhite spot on his tale fin. You can see it through the other side as well. It protrudes from the fin very little but is a noticeable "bump". Not round...just irregular shaped. Been in QT for 4 weeks with Mystery Wrasse and Flasher Wrasse. He eats well.

Sometimes it's more pronounced than others....

Any ideas? Thanks!