I had the same thing happen at about 2 weeks on some clones.Mine didn't look quite as bad. I put in some epson salt for mag dificiency and backed off my 400 HPS to 3 feet. It helped a little bit. A week later I got a 400 MH conversion bulb and lowered the light back down to 1 1/2 feet away. They look much better now, I got them in a small drip system. Got pic's in the hydro section. I wish I had of taken pic's of the curling. I have cal mag on the way just in case. what's your PPM and PH. I got northern lights and the dyna grow too.