Have you actually seen them eating the first bites? I've noticed that a lot of young fish refuse to eat anything that isn't moving until they've grown to half an inch or so. Is there a chance that they're being picked off by the other fish in the tank or getting sucked into the filter? As for water conditions, they're not particularly sensitive. I raised mine in a 10 gallon with a sponge filter & 30% weekly water changes.