the nutrients are too strong for little seedlings.
0.5ec or 500ppm is enough but it looks like there is more going on than just strong nutrients.
i looked at that system your running and it is either an aeroponic or nft gully system and im thinking that the seedlings roots are not growing out of that rockwool plug enough so you are basically drowning the roots with overwatering.
you would do better if you stand the plants in a tray and top water them with weak nutrients every day by hand until the roots grow through enough and you have a few sets of leaves growing out before transplanting to the hydro system. i found a couple of pics to help with root size before transplanting.