If you don't want to keep buying tanks, a 125 would be okay for several years, or a 180 would be even better. You don't state how large he is now, but a 180, with its standard footprint of 6x2, would hold him quite well until he reaches two feet. After that, a pond might be the best choice. Are you sure he is spotted, and not a Florida? Max sizes are different for these guys. A pic might help.