last time i checked they dont get too large..... i currently have a small pseudochromi (purple dottyback) in my 2 gal for the moment with my mantis, and they ahve gotten along so far very well. (though my mantis is small right now, just got him!!!) they tag teamed a few feeder fish today, but we will see how the mantis treats him as he gets a little bigger. Also, if you put the fish in first, and then introduce the mantis, it might not view the fish as food but as a part of the landscape. just an idea and an observation that ive had so far. lemme know how it ends up foryou!