Betta Fish Illness
Bettas like most fish rely on their slime coat to provide them a source of protection against paracites. When the animal becomes stressed or is touched by ojects that can remove slime coat, the scales are exposed. Which can lead to internal and external illness. Certainly bettas and other fish can get cancer. Im not really the one to ask when it comes to fish sickness, but the following is what i would do. Please refer to my forum, where the people can help you better than i can.
Paracites- For fish in general, paracites like velet and ich are common and treatable. Paracites can be spotted by the naked eye as little whitish dots on the external part of the fish. While their can be internal paratcites like worms, most of the common paracites are external. One of the best ways to treat is to use aquarium salt. Alot of freshwater paracites can not handle saltwater. So by giving your fish a saltbath, or just adding aquarium salt to the fishtank will be all for the best. Remember not to use table salt. The next thing to do is increase the water temp. Paracites will speed up development in heated water. This is good for ich. There are plenty of medications out there for fish and are effective on bettas. Also their are products out there, that can be used to treat internal paracites by coating the fish food with this kind of medications.
Fungal- Fungus can develop on open wounds. You can remove the fungus by hand with a net or tweezers, but the best thing to do is add salt and treat with medications.