Fungal infections in beta fish are caused by Columnaris which is a bacterial problem. Often the symptoms will appear as a light grey or white string like growth on most of the beta fishes body or extending onto their fins.
The appearance of a fungal infection is not uncommon for any type of tropical fish. It is when the beta fish has scrapes or cuts on its body that you should be more concerned for the welfare of the fish. Otherwise, the beta fish can return to normal health with the proper care and treatment.
The first step is to clean the beta fish tank. Fungal infections are much more likely to occur as a result of poor water conditions. You can read more about how to clean your beta fish tank .
Finally, to treat the fungal infection you should use Medications containing Malachite green. Malachite green is a completely safe and effective treatment in fresh and salt water on a wide range of protozoan, crustacean, and other invertebrate parasites of fishes.
Methylene blue is also effective.
Adding common table salt (sodium chloride) to the water, one teaspoon per gallon can help reduce the recovery time.
Beta fish are resilient but do not tolerate thermal swings very well. Betas are tropical fish and must be kept in tropical temperatures. Beta fish require regular, twice-weekly water changes in gallon water containers, a mix of foods (not simply dry pellets), and your interaction to stay healthy.
Most of the sicknesses or diseases with tropical freshwater fish can be attributed to their environment.
Almost always a quick water change, the addition of a bit of salt (unionized sea salt is the best, about a level teaspoon per five gallons equivalent) will solve whatever is ailing them.