I can tell right when they start swimming the difference between a blue and regular. These guys took a few weeks to tell the difference because they developed the stripes a little after they are born. I've also dealt with thousands of jack demspeys so I know what to look for.
Eventually I will be selling them once they are a bit bigger. I get far less of them in a spawn than I do blues though. Theoretically the percentage should be 6% per spawn.
Exactly. The only problem is if you breed siblings they pretty much all die or are deformed. So they need to be completely unrelated to get that small percentage of these guys.