why are my leaves getting so small and curly?

i have two plants growing side by side roughly 8 to 10 inches tall, both toppped once, they both got nice and full , then all of a sudden the new leaves became very small , and started to curl . They are both grown in 12 inch pots. 1/3 peat moss 1/3 vermiculite, 1/3 perlite under a 600 watt hps bulbs maybe 14 to 18 inches above them ?? please help

Hard to say, without pics. Are you sure the light isn't too hot for them? 14-18 is sorta close for a 600, especially if it's not air-cooled. Is it possible overfert, or a PH problem? Those could do it, too.

EDIT: Ooops, wrong thread.