Although generally you can't keep many fish with RTC, in a large enough enclosure with very little to no decore you stand a better chance. For one it ensures that the RTC will not have a specific spot to defend against intrusion, it also allows other fish the opportunity to see what the RTC is up to at all times. The fish should generally not be any smaller than 75% of the mass of an adult rtc to be housed with one. If the rtc is young you shouldn't keep anything with it unless it is at least 100% of the bulk or 100-125% of the total length if not very bulky. This means keeping them with other fish would require 1000 gal or more aquariums. Probably better than that would be 2000 gal or more. Also handfeeding your rtc will have them come to associate your hand with food, and they will react more to that than to other fish. It's hard to supress instinct but there are members on here who do it. Personally I have a rtc that lives with about a dozen other fish some smaller some larger than himself. He leaves them alone. I made a mistake with my CK because my wife didn't feed the fish while I was at work, and that just happened to be the perfect sized meal for the RTC, but he couldn't get it all the way down. Now I have someone feed him once while I am out at work for five days. That's the only fish he has ever tried to eat. He's been on shrimp since I got him. Blood worms then shrimp.