Possible magnesium defiency?

hey guys I was reading some of the plant issue sections on here and from what my plant looks like compared to a pic I just saw on here I think I might have a magnesium defiency but I'm not sure. Plant had been liking great and the discoloring just started happening. It had a couple brown spots here and there for a few weeks. Plant is in promix ho and ocean forest mix. Been following floranova simple drain to waste schedule with the exception of slightly less grow and bloom. I've keep my water and nute ph at about 6.3-6.5. I test my runoff and it usually comes out at 5.8-5.9. My last feed was 3/27. It had 553ppms going in at a ph of 6.5, and ppms coming out at 1490 with a ph of 5.8-5.9. I only see this on the top of the plant. Very bottom is lighter green but not much light is getting down there. I played with my ph the last couple weeks because my run off was around 5.7. My water before this feed was ph'd at 6.7, and before that 6.1. I should have never jumped like that but u live and learn. I'm kinda second guessing the way I test runoff and ph though. My tent is pretty full so when I feed or water I let the water level in the saucer get high enough to stick my ppm reader and ph pen in. But i started thinking about all the buildup in the saucer. Can that be giving me higher ppm readings and throwing my ph reading off or would the plant just absorb the nutes in the saucer. And not all the leaves look like the 1 in the pic but it seems to be getting worse. My 1000 watt hps is always about 18-24 inches from this plant. Tent gets up to 84 in the morning before I open a window and it drops to 79 for the rest of the day, night it stays 68-70. Humidity is 40-55%

Not sure about mixing Promix with soil, they both use different phs, so... I would say 6.5 is too high for the Promix, people usually go around 5.8 with that I think.
I don't Ph runoff, I don't think it gives you any accurate information, but greater minds can elaborate on that lol

I would mix a tsp of Epsom salt in a gallon of water and give them a foliar spray or two (at lights on), that will give them a mag boost.

Promix hp is not soil? What is it? Should I just do 1 or the other. I was doing around 6-6.1 and on another plant which I will add pic it wasn't looking good. My run off was like 5.4 when giving 6.1. I added ph'd nutes at 6.8 and in 5 hours it looked way better. But color has never came back to plant.