I'm guessing....light/heat stress? Some 400 watt metal halide bulbs, throw off too much UV rays. MH is not as forgiving as HPS rays. And different reflectors may cause hot spots.

If you can't raise the light more?...try offsetting the plants some....not directly under the bulb. I had this problem, once too.

have you transplanted lately?

distilled water is not the best thing to use either