Assuming basic saltwater husbandry... yes, I think that'd be a fine tank. I assume there is some circulation in there? I'd put 4" of fine sand covered by an inch of coarse live sand. Then I'd add a nice chunk of properly sized liverock for your mantis to live in.
Water changes are the key to a tank that small, maybe 1 cup or so a week.
HTH.
S !