I keep mine together, No problems, its a smasher and and they get along fine. Try adding an anemone to make the clown feel more comfortable... Also, you may want to get a decent sized clown of the more aggressive nature, like the clarkii, allardi, or tomato , etc. However, I kept a small damsel with mine as well, and the mantis was not interested in eating it either.. I also keep my mantis well fed on scallops. But even when I dont feed over a couple days, I think my mantis is more interested in whacking ME for food than her tankmate!