PH Control Problem

Im haveing truble controling my water ph. I am useing an Emily's Garden Hydroponic Syatem with and extra air supply. I am useing 3/4 strength FF hydro grow big and 2 drops of superthrive. Every time i check my ph it is going higher and higher. I have a PH control kit. I am trying to keep it around 5.8, but every time i check it has gone up again. I can tell my plants are not likeing the up and down PH levels. My plants are starting to get limp and my bottom leaves have started to turn yellow. They are only 3 weeks old and i dont wanna lose them.

If ya can mix the nutes with the water and then adjust the ph. The nutes will buffer and you will get a accurate ph after the water and nutes sit together for a bit.