Weird Spots and leaf color

Hey! I am a newbie doing my first grow in of Big Bud and Purple Haze in DWC.

4x4x8 grow tent. I use one air cooled 500W MH light for 2 totes, 2 lights and 4 totes total. Lights are 18" from the tops. I have (4) 10 gal totes currently filled with about 7 gals of water. My tap water is about 7.2 ph and 200 ppm. By the time I add 14ml Hydroguard to the water I end with 260-270 ppm. I am using GH Trio formula 1/2 tsp per gal, or about 17.5 ml from each...micro/grow/bloom bottle. I then PH down to 6.0. After PH down I end up with water at 6.0 PH and 660ppm/1320 EC. Temps are 74 degrees ambient, and 70 degrees water. RH is very low at 20-30%.

Humidity too low. Waht ppm are they? in low humidity plants drink more water and it can cause nute lockout, esp if they are young. just my 2c

From one newbie to another, I pH down to somewhere around 6, though be it litmus paper test I think I'm close, and start with tap somewhere around 130ppm and pH7. The ph down doesnt get me anywhere near as high ppm as yours went. I think it's 139ppm. Is your ph Meter off?