First DWC Grow and small Problem HELP!

Hello everyone and let me thank you in advance.
Like I said this is my first DWC grow and I got a lil problem. My plants have started to get spots of yellow all over the leaves, not the new growth. Within the yellow spots I can see these brown and allmost black spots starting to appear.
Also some leaves are kinda droopy, not much, and the leaf edges might be curling down and under. Im more focused on the yellow and brown/black spots tho.
I am using two 400 watt HPS lights with Son Agro bulbs.
I use GH 3 part Flora series nutes.
I also have not checked the PH in about 3 days and when I checked it tonight the average PH was at about 6.6 which I understand that it could be the problem.
Please if anyone has any thoughts it would be appriciated!

You should probably cover that rock wool with either hydroton, aluminum foil, or something to keep the light off of it and promoting algae growth.
How far away are your lights? R u using R/O water or tap? What's your ppm? 6.6pH is a little high, I'd bring it back down to about 6.0. The pH will tend to rise during early life and then it will fall as the plant get older.
I've had some plants curl too, but they'll get through it (see pics).

it could be one of two things the beginning of nutrient burn or it also could be windburn. make sure your not over feeding and dont have your fan blowing directly on your plants. good luck on the grow bro