When you are dealing with tumors-not much you can do except make the fish comfy....sometimes the tumor will grow to the point that is cause problems for the fish as far as swimming, eating...etc....and when this happens it is usually best to euthanize- but only if the fish is suffering and all treatments have failed...not a lot can be done for tumors....
Usually tumor develop as the fish ages, some genetic factor and environmental factor, immunity, sex...they can be benign or malignant...internal or external...
If you think it may be internal-usual sign/symptoms- asymmetrical in appearance...meaning one side is bigger than the other, usually will continue to poop with constipation they will not poop, sometimes appetite is affected sometimes not....S/S-vary with size, growth rate of the tumor and what organ it is affecting