feed once every 2-3 days. only feed chunks of frozen that are a third to half the mantis' own size. frozens can vary widely. white shrimp, squid, silversides, krill, etc... many can be found in an aisle of grocery (rinse grocery bought frozens after thawing). pick 2-3 different frozens to cycle between. soak the frozens in a supplement once in a while.
youll need to supplement with live food at least once a month. thin shelled hermits/snails work fine but the best show by far is a fight with a small crab. make sure the crab is small so the mantis won't get injured.
note that this diet is for a smasher mantis (most common). spearers have a somewhat dfferent diet.