it depends on the species of poly they don't grow to fast around 1''-2'' per month they can be picky at that size but it shouldn't be to hard to get it to eat try
blood worms
mysis shrimp
brine shrimp
tubifex worms
black worms
when a little bigger try
red worms/ red wigglers
pieces of shrimp like market shrimp the kind people eat
pieces of fish try cod ,tilapia ,carp ,catfish avoid fish like tuna and salmon there useually too oily
squid
beef heart
once you get a little food in it try to get it onto pellets try a few sinking pellets and see if you can't get it to eat them they do more then fine on the foods above but pellets help you save a little money and have more vitamins and most frozen foods are mostly water so it looks like there eating alot but really there getting alot of water