the tusk fish might have eat'n the gobie's, but I think that he might be good in your other tank, I 'm sure no one would pick on him. As far as a mantis, not all click that often, they might just be doing it at night while your asleep!!! I've seen a 1in. mantis take out a 2 1/2in. tomato clown out!! there's two type a stabber and a clubber. the stabber has a set of sharp claws that will almost tear a fish in half! The clubbers have huge hammer looking apendages that they use to "club" crabs and snails open to eat them. they both will also eat smaller fish and inverts. Keep an ear out!!