I'm in the navy. as such I'm only allowed to have a 20gal. After a while of thinking what is the coolest possible fish I could put in a 20 I came up with a mantis. I knew peacocks got to big so I got a green "spear type" mantis. He comes out every time I come to the tank but isnt eating the little shell fish I'm putting in. What should I be feeding him? Hes about 3-4".