This is the reason why high grade CRS so expensive, they are too much sensitive than others.
IMO, babies survial rate if > 50% is excellent otherwise in months time everyone will become professional breeder.
High NO3 is killer, if plants are healthy certainly can reduce certain percentage of nitrate and toxic but require very tedious maintenance routine to keep plants healthy without deficiency and algae, control lighting makes it easier. Keep water chemistry absolutely constant all the time especially having babies in tank; ph drift more than 0.3, temperature +- more than 1.5C, and oxidized trace are killer.
You should concentrate well treat your source water before water change, supplement minerals and find a way to remove toxic. I find that MineralPlus and Old Sea Mud rock in powder form once dissolved in water performs magic wonder. Again, no 2 tanks are identical, solution work for this may not work for other.
Good luck and happy shrimp keeping.