Flakes are good unless you have oveflows. In that case, much of the food is skimmed off the water before it has a chance to fall to the fish.
My YT is difficult to keep thick for some reason, but he'll eat just about anything. I feed pellets, frozen foods, and seaweed.
They mostly need their veggies, but meaty foods like krill, mysis, blood worms, and some brine can help round out their diets.
If you need help getting them to eat, soaking foods in garlic juice can help entice fish to eat. Other than that, just watching and learning from the other fish what to eat seems to help also.
Good luck.