like chris^ said fry do best when they have super clean water, they like it a little warmer than adults and regular small feedings of crushed flakes or baby food. It can be difficult to keep their water clean when you feed so often but hey...lol
In my experience stores wont usually give you credit for them unless they are a really colorful or unique strain and in good condition, but if you don't want them many will take them as feeders