It's still really hard to tell. Start with the salt and melafix. I would say to do the same treatment for either fin rot OR agression. This is what many people do for Bettas, and they can get some nasty fin rot. Keep an eye on it, if it keeps getting worse, then treat with some antibiotics. I really think it is agression, because of how you described it, and that your water stats are all good.
Fin rot is rarely fatal, and with the salt and melafix, if it does get worse, you will have time to treat it.