Since fish are cold blooded their body temperature (and metabolism) are regulated by the surrounding water temperature. When you are raising young fish many people elevate the temperature into the low 80s so that the fry will grow faster. This only works well if you are willing to feed them many times a day. This can have some bad side effects though, mainly lowered life span as the fish age faster because of the raised temperature.
If they are tiny fry I would have the temperature at about 80 and feed them a lot. If they are over 1/2 inch then I would put the temperature about 76 or 78 degrees F.