PH increases overnight even when treated. New member

First of I'm a relatively new grower, only about a year. Started with soil indoor, switched to soiless and now on coco. I've had some great success but am always trying to improve. I use week water and advanced nutrients ph perfect products and have never really paid much attention to ph into recently as I consistently noticed nutrient deficiencies after about 5 weeks of flowering. I started adjusting the ph of my water reservoir from 7/7.5 to 6 and then using as needed over the next few days mixing in my nutes to my water can before each use. Things seem better HOWEVER I measured the water the other day after it had sit for a nightand it was up to 8! Since then I've tried different types of pails for holding it, with and without air stones, with and without adjusting the ph and this keeps happening.
HELP!
I suppose I can just fill my watering can right from the tap and adjust right before use but it's much less convenient.
I haven't bought a digital reader yet just using drops from general hydroponics

This is my first post any input would be great

From yesterday: https://www.rollitup.org/t/why-does-my-phd-water-keep-changing-overnight.892032/