There's no single medicine that can be used to treat everything. There are so many things that cause illnesses - gram positive and gram negative bacteria, fungus, parasites, viruses, and even environmental problems that there are specific medicines that are needed to target the different pathogens to treat them. And things like BettaFix, MelaFix, and PimaFix are better as something to prevent a problem or treat a mild case of a disease, because there are the equivalent of herbal supplements, they aren't a medicine that will treat a serious condition.
From the sounds of what you're describing, your fish has fin rot, if the tail is flaking off at the end, or high ammonia if the color is just dark and splitting from the end toward the base. The best cure for both of these is regular [weekly] water changes of 25% if he's in a heated tank. If he's in a bowl, you'll need to do 100% water changes every 2-3 days and find a way to keep his water temperature around 76-80 degrees F. Adding a half teaspoon of aquarium salt per gallon will also help, and is about as good of an All-Purpose Medicine as you'll find. If the regular water changes don't help, or you're already doing the changes as I've described, you might need to add an antibiotic if this is fin rot. Maracyn 2 is a good 1 to get for this.