Ph rising and then crashes

Awesome set up man those are huge. I’d consider maybe mixing up outside the buckets first.. letting sit for 24 hrs before putting in the bucket. Those are massive plants for what looks like 5 gallon buckets, they’re in their prime time for sucking down water.. id be willing to be part of the drop in pH was simply from them suckin in water but not nutrients. When this happens the pH usually goes down. CO2 being brought thru the air pump into the stones can also cause pH swings.. Hope this helps! Those are huge plants, good work