Why does'nt my fish eat?

My fish is a very small thing. i dont no what type it is. it might be a babey. i bought it from a pet shop today along with 4 other fish. i broke up the food so that it was small so he could eat it because he has a small mouth and the food is big. but he wont eat it he will suk it and spit it out, why? is that normal? what should i do?