Good Hello!
I agree with namvet about the lights being too close. 75 centimetres away is not far enough for a 600 watt blast of light & heat.
Hopefully other owners of 600 watt lights will stop by here and tell you what heights have worked best for them.
If it's not too much bother, it might be helpful to test the PH of your rainwater...but I doubt that it is an issue. I think the plants are being stressed by the light being too close.
I personally would remove the ugliest looking leaves. It won't hurt the plant to do so. They aren't doing the plant any good.
Try not to worry too much about it. The situation is certainly salvageable. It is greatly to your advantage that you have not added any nutes so far. This means you know it's not a matter of nute-burn. (Deficiencies only seem to occur when the natural chemistry of the plant is knocked out of balance by the interfering introduction of additives/nutes to its growing medium.) IMHO, try adjusting the height of the light before you introduce additives.
Great grow karma to ya! :thumbs:
