Telling and enforcing are two different things. Have a serious talk with him, tell him how its going to be, and then actually follow through with that threat if he refuses to comply, bc he knows he can get away with what he is doing right now, so if he wants to continue to act immature, he has no reason to stop. My roomate raises fish too, however we don't mess with each others tank nor b*tch about what the other one has. We both have a lot of money in each of ours so we respect each other's property, and as a result, it has never been a problem.