Daily Ph spikes driving me crazy!!!

I am trying DWC for the first time using RO water and 1/4 strength FoxFarm Grow Big. I have 3 air stones (2 blue type, one gray).

I just changed the Res and set Ph around6.2.

I am very frustrated beacause every day I check the Ph it's up around 6.8+. I am going through Gen Hydro Dry Ph down like crazy.

What is causing these daily jumps in Ph.

I had these exact same problems when I was using FF in 5 gallon DWC systems but when I switched over to 20 gallons in an NFT system the PH swings were practically non-existent. Make sure you top your res off with ph balanced water everyday and my advice is to let your ph down circulate for a few hours and then measure it again to see if it's buffering correctly. At 7.0 PH water out my tap the grow big & big bloom would drop my PH down to 6.0 or so for a few hours and then it would begin climbing toward 7.0 again. I think this had to do with FF's buffering in their nutrients or lack thereof. If you check your PH 2 or more times per day you may not have as many PH swings. Also ensure you are topping off with PH balanced water everyday as I mentioned earlier.