Wait a couple days and see is good advice, if it's just habitual "I liked this other food I was used to, I don't want this new stuff!" pickiness. It may, however, be that the fish really just don't like the food at all and won't take it, or take it very reluctantly, even after going hungry for a while. If that's the case, it's often easier to switch back to your old food, or switch to another new brand of food until you find something they like better, than to try to force them onto something they're repeately refusing.