No matter what your ph will flux a lot the first 24hours of res change then 48 hours after should stabilize, at least that's how mine is, I put plain water in a 30 gallon res with 1 airstone, I pump it to get chlorine and anything that I can out of the water, I also then take a gallon bucket mix my solution up in it, to 5 gallon specs , I then ph 24 hours later I check ph and get back to 5.6 and let it rise up to 6.3 cannabis absorbs nutes at different ph levels, so letting it get higher isn't a bad thing, it is a good thing but just not to high, 5.5 to 6.3 for hydro and 6.0 to 7.0 for soil during flower I stay towards the higher side for example I am in coco and I am in flower and I ph to 6.0 to 6.2 for right now in veg I always was at 5.8