When they are still small flakes work, free dried or frozen bloodworms, and get them on hikari pellets, they should take them eagerly. Also go a variety of shrimp, tilapia, catfish, or other white fish are good to feed them, also earthworms. The more you switch it around when they are younger the easier and cheap it will be when they are bigger.
I dont understand when people say give fish guppies or goldfish as "treats." Its more of a treat for the owner to watch them eat live.