What has my young male betta done to his anal fin?

[On_a_dishy]

He has been jarred for several days now, and I don't remember seeing any injury when I was gazing lovingly at him then. But now he appears to have a hole in his anal fin. I've given him a blue meth bath and he's as perky as ever, but I'm wondering how he could have done this to himself. In my limited experience, a nipped fin is an obvious bite, whereas this seems to be a hole!

 

[EllRog]

He'd have bitten it out of himself unless it's been caught while performing a waterchange