I had/have a similar issue with a single CPD after having my group for several months. I'm not sure mine has fin issues but was very obviously not getting enough food. I let it go for a couple weeks but finally moved the fish to another tank in case whatever it is spreads. I moved mine in an unheated tank that I toss plant clippings into. I've been feeding small amounts every few days...use NLS here also but now have live micro worms (thanks Dad) so I offer those also. The fish is still kicking, looks a little better but hard to see with all the plant clippings. I would move the fish especially if you think it has any sort of rot/fungus.