first you should really have a Metal Halide instead of an HPS if youre in Veg cycle. (18hrs) how big is your setup? it may be that 600w is generating too much heat and you need a 400w , or you can get a strong enough exhaust fan that removes plenty of hot air (air cooled light too) your curling can be coming form them being too hot you just back off the light more and see it that helps, it could just fix it.