I just read a response....and last question....when you put the eggs in tank with m.blue, heater, air stone....and WAIT...if you could not use your main tank water or tap water, what would you use? All my adults live. If I could get these guys past 3 days swimming, hopefully they would adapt to whatever is wrong with my water. So....spring water? Distilled?
It's starting to sound like it's a water thing keeping you from moving forward. I've never seen a fry revert back to a wiggler with the sticky thread once it had become free swimming. Since we are not talking about wild or near wild fish, this could be a genetic thing? Fry don't adapt to bad water conditions like adults do, they just die.
In order to find the problem, you have to eliminate things one by one.
Here's the next experiment, when the eggs are laid, as soon as possible, put them into distilled water you buy at the store or rainwater you collect yourself. (Within 15 minutes of finished spawning would be ideal.) Don't distill your house's water. Make sure the PH is low as well (6.5-6.0). Use a heater, airstone and M. Blue. Do not do any water changes on the eggs. Use a small carbon bag to remove the M. Blue once the eggs have hatched, NOT BEFORE. Once the eggs have hatched, NO WATER CHANGES on them. If the fry are not free swimming by day 7, the problem is the fish themselves.
Now, if the fry are free swimming, you will need to feed them. Baby brine shrimp is my food of choice. Now that you know how to hatch those, you'll know to set them up on day 6 (if yours hatch out in 24 hours. adjust accordingly) so they will be ready. As a first feeding, I feed only the amount of live shrimp that fit underneath my index finger nail. ( I have short nails
) You should see the fry eating the shrimp or the after effects ( the orange belly.) Once you see them eating, you'll know that the water you used is the right water for the eggs to hatch, fry to mature and free swimmers to eat. Anything that goes wrong after this is from what you do next because THIS water is good. You may want to get a few feedings into these fry before you move them. This way, they should be good and strong and able to make the move. If you have the space, when you move them, you can split up the fry into 2 tanks where one is an exact duplicate of the fry water and the other is the water you are currently using for the other fish. If both fry tanks survive, the problem is not the water. If you lose the ones in your water vs the store bought distilled water, you'll know it's your water and you will need to experiment on whether distilling your water has any effect on the fry.
Unfortunately, there are only really 2 choices as to what's wrong: The water you're using or the fish's genetics. You can't test for the genetics so you have to eliminate the water.
Keep us posted
PS: If you use rainwater, check and adjust the PH (as described above) and I use PRIME in my rainwater "just in case" because my water runs down a metal roof before collection. I've had no bad effects on the fry.