[ao]

Apparently fungus is always present in a tank. you can treat the little guy in a quarantine container or float a container in the larger tank. pre measure the salt in a 1 gal at 1 tsp/gal and use that for your bettas water. Im not an expert on fungi, so you should wait for someone with experience to come along I know that warmer temperature will only encourage fungi. good luck!

Any pictures? it usually helps with a diagnosis