I had a mantis in my tank, he was a smasher which means he ate anything with a shell. He ate 6 hermits, 4 snails, 1 horseshoe crab, and 1 scallop.
When we moved "his" rock was placed in the lionfish tank. Poor little mantis...... <img src="graemlins//uhuh.gif" border="0" alt="[U-Huh]" />
Slashers, BTW will eat fish instead.....
It's a very good idea to remove them from your tank as quickly as possible. Some people do keep them as pets in seperate tanks though.