Fin rot!!! Help!

A guppy in my tank has been suffering from fin rot, I think. I have 3 guppys and 3 neons.

The guppy's tail looks mildly shredded horizontally, and it seems to be getting worse. I've raised the temperature in the tank to about 80, and have been treating with Melafix for about 5 days now, but still seems to be getting worse.

Any advice on what I should do?

The tank is due for a water change today, as well.

Thanks!!

(I've read online that Tetras are also tail-nippers, but I haven't seen any malicious activity from them, yet.)

10g tank. The neons seem to be doing fine.

And really, turn down the temp? I always thought higher temp would kill the bad stuff..
Like how the human body raises temp when you're sick, etc. Weird.

But I will lower the temperature to regular settings.