It is said that an ounce of prevention is worth a pound of cure. While this is definitely the case with bettas, once they become sick, their condition can deteriorate quickly. That is why you want to be ready in advance with all the tools you will need if your fish contracts any of the many betta diseases.
You might think that you can always head out to the local pet store if and when you notice your fish are unhealthy. There are two major problems with this strategy. First, by the time symptoms appear on your fish, the betta diseases causing those symptoms have already been growing, and becoming stronger, and your fish weaker. This means a longer recovery time, and a smaller window for treatment. Second, the store may be out of whatever meds or tools you need. That could spell disaster for your betta.
The first thing you will want to purchase for your betta first aid kit is sea salt, or aquarium salt. You do not need much, and it is very inexpensive. When treating bettas in a recovery tank or bucket, generally a couple tablespoons per gallon of water is needed. You will also find that placement of a tablespoon per gallon of tank water does wonders for your fish during tank water changes.
Buy a one gallon bowl or jar for holding and treating fish. When you remove sick bettas to this tank, use the water they were already in and acclimated to, as to less the shock of moving. Keep some antibacterial soap on hand for yourself, as you definitely do not want to spread disease to your healthy fish after handling sick ones.
Buy rubber gloves and a scrub brush that you use specifically for tank cleaning and water changes and nothing else. I was at a friend's house once, and suggested that his tank was behind time for a cleaning. He promptly went to his kitchen and returned with a scrub brush. Yikes! No matter how hard you clean, you may transfer chemicals to you betta's tank. Only use tools that are specifically for your betta tank.
Betta diseases are all different, but a few chemicals treat many of them. Tetracychlin is a good all around treatment for many betta bacteria, and can be found at most pet stores. The more difficult betta diseases need to be treated on an individual basis, but some of the better meds for these are kanamycin and ampicillin. As always, consult manufacturer's labels for proper application.