Hello fyouston,

I think what you read might be the whole range of hydro. First see what the mfg of your nutes recoments for RW. But I have found that hydro in soil the ph should be about 6.3 (+/-.2), in pro-mix about 6(+/-.2) and for rockwool 5.5(+/-.2). So I too think your ph is too high.

Your temperture while your lights are on seems high to me also, 24-25c is about maxium without additional co2. Your high of 28c will slow growth for sure, this might be a factor in the yellowing leaves tho it could be other things. (maybe overwatering, but not sure about that)

I usually use PPT (a little more accurate then PPM) but I believe that a level of 2000 PPM = 4 EC. And 1200PPM (a common nute level) = 2.4 EC. Is it possible you've left out the decimal points?

That's all I see wrong at first look, hope it might help you. Good luck and keep us posted.