runoff ph too low, maybe too much fulvic acid?

So for a little set up. I grow in 4x4 rockwool cubes and put those into a 3 gallon pot and flower like that. I have been doing the same system for a little over 3 years now, and this problem just started happening a couple of months ago. I flush every week to record my runoff ph and ppm to make sure I am not overfeeding or anything like that. Every table now is getting low ph during weeks 4,5 and 6. It shows up as a cal or mag deficiency but I have had the same strain running on the same dose of cal mag and this never used to happen. I recently switched to another fulvic acid called uptake by Cyco, which has a lot more fulvic acid than my last product. Could too much fulvic be causing this? I PH my waterings at 6.0 and my week 5 and 6 runoff usually comes out around 5.1 or something pretty low like that, which would explain the cal or mag deficiency that the plants are showing because the medium ph is too low to absorb. Any ideas or help for rasing the ph?

I know this thread is old however I'm going to respond to it anyway. You are having problems because your starting pH is too high! You have to use 5.5 pH with grodan rockwool.

why would high pg make the mdium ph low? and its only happening week 5-6 and we ph at pretty much the same all the way through

Nutrient and acid accumulation in the cubes will bring down the pH over time and cause leaf scaring! Suggestion would be to to take the 6 week mix and use as a flush! I use 5.5 pH with Gen Hydroponics in RO water with a little cal-mag+.

Even with starting pH of 5.5 the cubes swing high on me to about 6.1 pH when I check them. So I feed with every watering to keep the nutrients available at 5.5.

What are you putting the rockwool cube into as far as media in the three gallon pot?