Any green leafy vegetables. they love those. Romaine lettuce is one of their favorite. You can sometimes just ask your local grocery to save the wilted unsellable ones for you and you can get them for free and feed it to your fish.
essentially anything that they can swallow, some have certain tastes tho.
strawberries seems to be a favorite. and halved grapes. sweet S#!T mostly, they have a huge sweet tooth. chrickets and mealworms seem to be another hot item