i housed a 12 inch mbu with my 12 inch rtc, who knows later on.. i have seen video of a plastic iguana being sergically removed from a rtc.. so my guess is standard catfish rules, if it fits in its mouth ... chances are its lunch, so get the puffer larger than your red tail will be 3 months in advance, it will give your mbu a chance lol, but it is an unwinnable race, eventually a rtc could eat all kids of stuff...