It's no surprise to me that people are having trouble with the health of the fry. I have culled every batch of mine so far because they are weak and drop like flies after about two weeks. I think I know why, and I'm going to be testing that out on the next spawn. If it turns out to be right, I'll document it here.
I think he posted in another thread that it was gill flukes. Don't quote me on that I can't seem to find the thread.