Please help

Ok, all seemed to be going pretty well, untill a few days ago when the tips of the lower leaves started crisping. I'm very woried.
Plants are about 3 weeks old, snow white, under a MH 400W lamp.
The edges of the lower leaves are going yellow/brown/light green and going completely dry (necrosis I think??).
Anyone got any ideas what it might be? Could it be due to the light being too close? (had it bout 20 inches away.)
Got a pic but its too big to post. Havent used any fert. Watering bout every other day.

Thank you very much to any1 with any suggestions.

Tom

 

ok, think I got the pic to work....

 

Thanks. Just given them very watered down fert. How long do you think it will take them to show signs of improvement if it was lack of fert that was the problem?

Tom