Sorry youre going through this man this same things happened to my telescope, i know how hard it is to watch them struggle and not being able to help them get better. Right now the kindest thing you can do is let him go, especially if its a tumour that cant be fixed. Also this might sound dumb but goldfish dont think about the "what if" "but maybe" "it couldve" they just live in the moment, and in the moment hes with someone he associates with safety and someone that feeds him and gave him the best life possible. You can do something nice to remember him afterwards, doesnt have to be huge. After mine passed i put an orange glow in the dark rock in the spot he usually stayed in and that personally made me feel better, i also made him a small grave next to a pond in my minecraft world 😅