You should use a different nutrient. I use GH's Maxi series of powders. They are pretty cheap, contain cal/mag elements, ph buffer, and everything needed for that stage of growth. There are 3 bags. One for veg, that's for while your in 18/6 light, the flower bag for 12/12 light and finally a kool bloom for the final 2 weeks as an additive. I also use coco as my medium which is ph neutral. With the two combined I rarely adjust ph ever. I thought I was gonna need to adjust up once, but I discovered I really needed a new ph pen. LOL

I have 2 separate 15 gallon reservoirs that I maintain, one for veg and one for flower. The veg side tends to need a little more maintaince in terms of ph for some reason but on the flower side all I ever do is continue to add more nutes and water. The veg side needs a lower ppm so I cant add as much nutes therefore that ph stays a little higher as I refuse to chase ph unless it's way out of whack. Usually that's the sign to change water for me anyway. It takes about 3 weeks before I feel the need to change the water.