I'm so sorry that your betta is sick. We'll probably need a little more info to find out what's wrong...
First of all, I'm not sure what you mean by a hot water heater... do you mean an aquarium heater? That's really the only thing you should be using to heat the water in the tank, as it needs to be held at a stable temperature, not just warmed up and then put in the tank, where it will cool down again.
Second, have you tested your water for ammonia/nitrites/nitrates? pH? You will need to do this to be sure that it's not basic water chemistry problems that are making your fish ill.
Finally, there is some question about Bettafix being safe for bettas or not. But even if it is safe, its active ingredient, melaleuca, is only an antiseptic and can't treat bad infections or help with water toxicity issues.