Ok, because Finnick's tail edges are never bloodied/black and his tail-disappearing act seems to happen over a span of a few short hours and very suddenly (in this case of today, like 3) I'm assuming he's tail-biting.
The problem is, his tail starts to regrow (clear edges, seemingly becoming the new fin) and then after a week or so it's all gone one day, and part of his "good" tail with it! At this rate he won't have any tail left within a few months if he doesn't stop.
This is beginning to frustrate me to tears. Now it's not just the caudal fin, but the anal fin as well. I don't know what to do! How do I get him to stop?