Need an Expert or a Magician

I am at a total loss. Let me explain.

Been growing for several years,
using ebb & flow
Bagseed growing in rockwool
Well water that is about 75 PPM
GH nutes taking water to about 250PPM
PH around 6-6.5
Fouros for 18 hours a day
1 watering (30 minutes) every day

OK, seeds sprout and start to grow until they get about 1-2 inches high. Then the lower leaves start to turn brown and the plants slow down their growth. I can't figure out what has changed. I hope you can give me some hints.

some pictures?

If you're growing in rockwool you are hydro, your ph should be 5.8, nute strength at 250 seems ok for such a young plant, pics would help.

I suggest it's either pH or a root problem.

PH or beginning of root rot but need pics. Id say PH needs to be lowered gradually so you don't throw her into shock.