Rearing tanks for your fry need to be bare, with no gravel and other unnecessary clutter. You want it to be easy to keep clean. Growing fry need to be fed heavier and more often than normal adult fish and therefore you'll need to make more frequent water changes. You need to use a good filter, but one that doesn't cause too much turbulence that would knock the fry around. I recommend a sponge filter for fry tanks.
The size of your rearing tank will depend on how many fry are going to be in it at any one time. For example a small batch of 10 Killifish fry would only need a one gallon container to start with, you want to keep the fry in close proximity to their food. But a batch of 200 Angel Fish fry would need at least a 10 gallon tank to start with and would need to be spread out into more tanks very soon, as they start growing.
The main rules for rearing tanks are Cleanliness, Heavy Feeding, Frequent Water Changes and Room To Grow.