Yes Betta do not tolerate MelaFix well or Pima Fix at normal doses. I'm not sure if it is both or just one they don't tolerate. But I'm guessing both. BettaFix is at a reduced percentage, because they don't tolerate it, and it kills them at the higher doses. The medication issues are one reason, I have become sceptical of keeping them in a community tank.
Betta are prone/suspetible to fin rot, so it is good to treat wounds, and be sure to take extra care with the water until he heals. Probably keeping the water extra clean is enough, but I also treat.
I remember that about Rocket's save from the wicked danios