Are you feeding nutes?
How close are lights?
How often are you watering? cuz that compost looks REAL WET! and what compost is it?? Cuz some composts are packed full of nutes and that can be real bad..
Do you have any perlite in the soil to aid airation to the roots?
Why have you bunched the plants in one pot??
Looks like it could be nute burn, if you are feeding nutes, back off, Flush the pot well with PH7 water,leave it to drain and put it back under the lights.
You might wanna seperate the plants asap, as itll be a bitch to seperate me without damaging the roots if you leave em, as the root systems will intertwine and they could strangle each other...When you repot,mix in 25% volume of perlite, to aid water retention and airation of the roots.
Compost looks VERY wet,This could also be your problem, it causes root rot and mold amongst other things, and doesnt allow the roots to get vital Oxygen...Let the soil dry out, check every day, poke a finger 2" down in the pot, if your finger comes out moist, theyre good for another day or so.
What compost is it?? If its stuff like Miracle gro, that can suck for plants, when you repot, try a tomato style organic compost medium or somthing similar
Finaly check your lights arent too close, could be light burn/heat stress.
Its a bit hard to tell exactly without knowing your exact setup.
