I've tried almost everything with Peanut, my teal doublefin, bettafix to a whole new tank with exciting things, and I've never solved the mystery with his strange depleting fins... things are getting better now that it's been a few months luckily. There are clear parts of fin on top of his damaged bits that I assume are new stuff growing back? But it's not 100% still. My conclusion is that he's tail biting but what are other possibilities and solutions?
Besides a few rough fins he's spunky and feisty as ever and appears to be in good health.