I've read a lot about Bettafix. Some people claim that the active ingredient in it can harm Bettas, but the thing about it is that it is SOOO diluted in Bettafix that you really don't have to worry about it.
Unless you dumped the whole bottle in like a two gallon tank or something, I wouldn't concern myself with it at all.
If you DID do something like that, just change about half the water and replace it with non-bettafixed water.
I'm starting to think that mine had finrot when I got him and I didn't even know because I had never had Bettas before. I've had him for about 2 weeks - I didn't really 'treat' the finrot at all and it appears he has already grown a few milimeters of fin back. It's hard to tell, though. I am just guessing that from comparing him to pictures of him when I first got him.
That was a lot of hims.