4 week old clone with curling rigid leaves.

The leaves start to curl sometimes. They are under a 600 watt hid light 24 inches above the plants.

u got any airflow goin?

Yes I got an 8 inch inline fan pumping air outta the room and cooling the lights. I insulated the room well though to keep the heat in so maybe the room is to airtight?

whats the temp at the top of the plant? Is the room a small volume?

"Sometimes" as in the same day you feed your plants?

Borderline moisture stress or overfeeding is my guess.

This is minor though, and shouldn't affect your end product.