2nd Week of flowering, PH get jacked within hours. Is that normal?

Hi,
My kids are in their 2nd week of flowering, looking most excellent. I have a homemade aero system using Humboldt nutes. I change the water weekly using ro water. I mix my nutes, I'm kinda scared here as I almost wasted them once, so I am at about 1000ppm. I adjust my water to a PH of 5.8, check it within an hour or 2 and it has creeped up to 6.2, and it's the same thing all day. By the end of the 12 hour sleepy time, the PH is over 7, which kinda freaks me out. This is my first aero grow, is the PH being all jacked up while flowering normal? Thanks!
 
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