My son (OK, and me too) is frantic because we have 2 new breeds but they are still "teenagers" and their health is already into the yellow. There are no diseases or dead fish present, and we have done the research environment (level 2). Any suggestions? I don't think he has the thermometer in his, has anyone been able to tell if that makes a difference? Other fish in tank are healthy, it's just the babies.

If the young fish are of the rarest varieties, they will not be able to remain completely healthy until you have fully researched Fish Environment (Level 3). If their health gets into the red, and you have enough money, you can buy vitamins and drop them into their tank. This will boost their health enough that they'll probably survive into adulthood (at which point you can sell them).

Also make sure that you're careful not to overfeed them. Feeding your fish too much or too often can harm them. If you feed them before they are hungry (about halfway down on their "Fed" meter), they will lose health each time you feed them.

Thanks so much! I didn't know about the feeding thing, we definately feed ours too much. We will try the vitamins, and assume that they probably are too rare to survive until we research environment again.
My real fish tank is dirty and neglected! This one is much more fun!