Bump on base of fin?

[Sssea]

My oranda has a strange bump on his base of his tail fin. Today is the first day I've noticed it. It's definitely not ich, it looks more like a raised bump or pimple or something. Any ideas?

 

[Sssea]

Here's a better picture to show how raised it is.

 

[Sssea]

Anyone? It seems to have gotten bigger throughout the day.