If he is one day old, he should be receiving special fry foods. Fry have different nutritional needs, just as puppies and kittens should not be fed adult foods until they are adults.
You can substitute one fish food for another on occasion, if the fish accept it. Say, if you run out of the appropriate kind. However, you should not do this continually. A Betta, for example, has higher requirements for protein than Guppies. He can survive on Guppy food, but proper Betta food is optimal for his best health.
Some fish are just fussy eaters. Most Bettas will not touch flake food, a few prefer flakes over pellets, and some want frozen foods. They also get bored with the same old thing day in and day out, year after year. You should feed a healthy adult Betta a diet consisting mostly of good quality Betta food, but a few times a week, substitute blood worms and/or brine shrimp for his usual food. He will enjoy the variety, and it is good for him.
One day a week, feed him nothing. This is essential for a healthy digestive system. I choose to do my partial water change on that day.
Hope this helps! Good luck!