First grow and i have spots. PLEASE HELP

I have a 4'x4'x8' grow tent with two eco growers that hold 6 plants each. My plants are 3 weeks old and are on a half nutrient diet. I use general hydro for my nutrients. The spots started on one plant on its lower leaves and now its spreading a lot one the first plant and very slowly onto others. PH was a little high at 6.9 so i lowered it. PLEASE HELP MY BABIES:shock:
 

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