If the curling leaves were all over the plant, as opposed to just the tops, then a diagnosis may have been a little different. Nutrient lockout, salt buildup, unfavorable pH, etc. But since you're not seeing it on the rest of the plant, most likely it's just the tops responding to the heat. Raise the light, or bend the stems slightly to get them away from the direct heat. Sounds to me like this should fix your problem.