I've decided to list some sites that are clear with their ways of identifying diseases and how they go about treating/curing them. So, check these out and see if they will help with your situation. If you can't find a solution to your betta disease problem, I suggest you end their pain & suffering by euthanasia. The way to do this is either to give them an overdose of anesthetics (if you have any) or just to put them into a plastic bag w/their water and put them in the freezer, where they'll go to sleep quietly...
Aquatronics Homepage: Disease Diagnosis
Faith's Betta Talk
For some quick steps for some diseases look below:
1. Ick or Ich-Use anything w/Malchite Green and raise temp to 83.
2. Fin Rot-(depends on stage though but...)Melafix with aquarium salt (1 Tbsp./5gal).
3. Lameness/mal-nutrition due to poor water-use Betta Max!!!
4. Non-Meds methods-Salt Method: Add 1 tsp. Aq. salt to 1 gal. of water for 3 days. (Total at the end of 3 days will we 1 Tbsp./1 gal. water. (usually works just about on any disease; fish are fine)
*I recommend to use Colloidal Silver (@GNC) for nutrition supplements during disease*