If you notice a tumor on your fish, the first thing to do is consult a veterinarian who specializes in aquatic animals. It's important to diagnose why the tumor has appeared and determine if it is treatable or not. Depending on the type of tumor, your vet may be able to surgically remove it or prescribe medication that can help fight off any resulting infections. If surgery isn't an option, then providing supportive care by maintaining optimal water quality and proper nutrition can go a long way towards keeping your fish healthy while they battle the disease. In addition to these measures, you'll also want to quarantine any other fish in your tank as tumors are often contagious and can spread quickly throughout a tank population. Finally, keep an eye out for changes in behavior or appetite as these could indicate that something more serious is happening with your fish's health and require immediate attention from an experienced aquatic veterinarian.