can you identify the frogs? Different species do well at different temperatures. Also, sorry I don't much experience, but it does seem that froglets are vulnerable, fragile things. Did you use declorinated water? That tends to be the best, since distilled water lacks minerals. As far as letting them go in the spring, I'm guessing they'll be used to captivity by then and might not be able to hunt well, but you might want to ask someone else's opinion about that.