Finrot, Tailbiting, or the Filter!?!

[Elaina]

So one of my boys, Fonzie, is either periodically chomping on his tail, getting finrot, or getting his tail stuck in the filter. In the 6 months or so that I've had him I have found him with a shredded tail 3 times. The first time I thought it was the filter, so I slightly covered up the intake so he couldn't get sucked into it. But since I've "covered" it, it has happened another 2 times. Each time it looks like he's suddenly a crowntail, instead of a superdelta. Is there any definitive way to tell whether he is tailbiting, or getting finrot? I'm at my wits end, he has such a gorgeous tail!!