Well, maybe I should have included a little more information and asked more questions about what type of tank he had. Sorry. I was only going by my own experience because I have sold someone that I know a volitan lionfish for their reef tank because they could not get the mantis shrimp out of their tank. They tried everything from traps to ripping apart their tank, and nothing worked. After I gave him the lion, he did not lose a fish afterwards. He never saw him eat it however, but the shrimp stopped his aggressive behavior I guess because fish stopped disappearing. Thus, maybe it would hurt its mouth a little, but I don't think a bigger volitan would have a problem with a small mantis.