Problem might be in the water. Are you changing the water in the fry tank?
Might be you're shocking the fry with too massive water changes. If you've really got to get out the detritus that's fine. However, be careful introducing the replacement water. i.e., slowly introduce it, maybe a trickle from an air line.
How about the food? Are they eating the food you're introducing to them? Could be they're starving? Try different food, perhaps something more appetizing to them. Maybe their present fare is just to big for them to properly process? Try something smaller, from green water to micro worms, newly hatched brine shrimp, etc., etc.
After your success be sure to share you experiences with us.
v.b.