Some nutrient mixes include pH buffering. My make-up water (plain ol tapwater) is about 8.4 before adding Canna nutes, 5.8 after.
How far your pH wanders depends mainly on stuff in your system and how 'hard' your water is, i.e. how much dissolved limestone in it.
Rockwool is known to have a bit of limestone dust in it which should be flushed out prior to planting clones in cubes. pH in rockwool based systems can jump up for the reason of limestone traces not fully rinsed out, meaning the nute tank must be corrected downward with phosphoric acid (pHDown).
'Hard' water must also be corrected down each time the tank is topped up, for the same reason.
I run my tanks at 5.3-5.8 because of the slight alkalinity of the rockwool floc I use as a potstuffer.
I flush rw cubes in running water and then soak in 5.0 water for 24hrs, remove the pre-soak water with a salad spinner and then water with grow nutes at a weak 400ppm @ 5.3-5.8 with a before putting cuttings in them.
