Split down caudal fin

My VT, Prince, has a big U-shaped tear in the middle of his caudal fin. It's not rot, and I think he might have ripped it on a plastic plant in his tank or something. I'm going to retest everything with hose and for now, he's in an undecorated, unheated QT 1g bowl on my desk temporarily while I sort things out. He shares a divided 10g tank with a DT boy who is recovering from severe rot attacks. This tank is heated 80 degrees F. Filtered with a Tetra Whisper 10i. Water changes 75% weekly. The split appeared today.

I am planning to treat with AQ salt.

If I quarantine anyone, I won't be able to heat any fish's water. That is my problem. I have an unheated 2.5g and an unheated 1g at the ready.

Also I've been thinking to keep Prince in the big tank to heal and instead quarantine my other boy on the other side of the divider, Monkey, who still has pretty bad fin rot damage and I'm not sure if it's gone or not. But like I said I don't have the spare heater for the spare tank.

I was planning to clean out the tank completely 100% today and get rid of the gravel (because it's so annoying to clean), but I saw the split and I wasn't sure what to do.

What should I do? Please help.