Can't help you at all 'coz my last batches of fry are not growing as much either. I can't see how 20 fry in a 10 gallon could be considered overcrowding. I would think there was plenty of room for them at the keyboard key size.

Could your tank have been kept cooler perhaps? Mine has been 1-2°F cooler over the last few months because it has been winter here (and that's gone thank goodness).

Is this your first batch of fry in that tank? I ask that b/c I was starting to wonder if it is the same as for my goats - the last ones born during a season never grow as well as the first born. I've always assumed it was due to a build up of bacteria and other unfriendly organisms and that the kids, despite being healthy, have to use their immune system more. Maybe it's the same with fish???