When you notice your fish is ill the first thing you should do is make a note of all the symptoms. Do you notice color loss, bloating, inactivity, ulcers or lesions, redness, bumps, specs, filmy coating or bodily distortions, etc?
Fix any problems with the water right away. Poor water conditions are the leading cause of fish illness and all the medicine in the world won’t help him if he continues to live in bad water. Remember, most of the problems in water have no color, smell or texture. The only way to know is to test.
Next, compare the symptoms with the list of common betta illnesses to determine what the problem is. If it’s minor, then the betta can usually heal himself without the use of harsh medications. If you feel it is more severe you may need to medicate. Be sure to follow the instructions on the medicine package carefully. If you are not sure how to diagnose your betta there are fish forums and news groups where people are available pretty much 24 hours a day and they can help you. One of my favorites is Aquamaniacs.net
When you keep several fish in the same tank and one becomes ill you don’t want to medicate the other healthy fish. This could actually weaken their immune systems and lead to additional illnesses. It’s always a good idea to keep a spare tank around to use as a hospital tank. It could be something as simple as a 2.5 gallon and a 25-watt heater and air-stone. Bettas don’t require air-stones but because many medications deplete the oxygen in the water it will help to make your betta more comfortable by allowing them to surface less frequently for air. When all your fish are healthy you can easily clean out your hospital tank and tuck it away until you need it again.