Advice on tail biting? It's getting BAD fast. What has worked for you?

My worst nightmare came true and now my Dude is biting his fins like they pissed him off. Parameters are all good at 0/0/<20ppm. I thought maybe he didn't have enough room to swim so I removed one of his water wisteria plants to free up space and put back the hide that he usually has that I'd replaced for a more festive last week. I did a water change, about 20% with added salt to help him heal, literally two hours ago, went out to dinner and when I came back another HUGE chunk was missing. My water change schedule is 20-30% a week. Last week during his water change I forgot to plug his heater back until the next morning in so his tank was cool for a few days until it warmed up. These have been the only recent changes in his tank and I've had him since September.

What has worked for you to stop this behavior? What can I do? Would letting him flare at himself in a mirror help to distract him or anything? I'm so upset.