Can you tell us what type of plants? Or is it a single type of plant? Is this happening in a localized area of the tank (eg. one side of the tank or at one height) or is it spread out. A picture would definitely help. I can't quite imagine what you mean by "curl" (bend? arch?).
hi Alym, if it curl and the leaves r deformed but otherwise a healthy plant it might be ur GH is low, aim for 4 to 5 gh. I had this problem on my cube tank with MH and I find dosing EQUILIBRIUM OR Tom Barr gh booster fixed this problem.thanks