When researching for Whiskey's tumor, I read an article where a betta fish had fluid filled cysts and they drained them with a syringe. I don't know if there's any vet that would be willing to do that. It would be tough to find I imagine. I was thinking out loud, I guess. But the picture did look a lot like Alexander's tumor.
And yes, fish can and do get cancer. True sharks are the only aquatics that don't get cancer.