Not systemic through entire plant, so doubtful it's salt or micronute build-up.
Since mainly the tops, likely heat and/or wind-burn. Fill-out the troubleshooting form. Likely it will show us exactly what's up.

So what's going on with those poor seedlings? In way too big of pots for their age. Makes watering properly a bitch and is wasteful. They are too far from the lights, they're stretching too much.
One seed, one pot. What's that mess in the middle, lol?

Seedling Stretch Fix
Twords the bottom of the post.
Rusty Trichome Reviewed by Rusty Trichome on . Leaf Curling, Turning Brown 400w hps flowering Any advice? 400w hps week 5 into flowering nutes every 3rd watering Rating: 5