So last year this lady who used to live next door had a goldfish she didn't want, and I said I might take it, I just wanted to inspect and quarantine it before putting it in my pond, but instead of giving it to me to quarantine, she came and put it in my pond when I wasn't home. Now the fish has a tumor growing in it's back. Tumors aren't usually contagious, but I'm thinking about bringing in the fish anyway, I have an empty 40 gallon tank I could put it in. I have never had a goldfish with a tumor before, how should I treat it? I'm not sure if the tumor is cancerous or not, the tumor looks pretty big. If it is cancerous, I may just have to euthanize the fish.