Constant ph value of rdwc system

Buy nutes that are pH buffered. I never have to use ph-down or up. If you use RO, that will be more likely to drift but won't with a ph-buffered fertilizer. pH is a resulting ratio of OH- : H3O+ that result when various acids/bases dissolve into water. Larger numbers with same ratio will be more resistant to drift.. fluctuations becomes small percentages of the whole. The other problem is that plants do selectively transport molecules into the roots - it's not all by diffusion and even that does not gaurantee 'everything' dissolved in the water can anter, so what is left behind can be in a different ratio than the formual with which you fertigated. This takes time. Refresh water more often and should take care of that. Microbes could feasilby do the same thing -- change the ratio of solutes that exist in the water and therefore the pH could swing. i find with my rez, i can delay cleaning maintenance significantly as long as the plants are drinking it up fast enough and after 3 fertigations i immediately refill it. It never rests at less than 50% full for more than 24 hours. It sits at 25% full for about 15minutes before i refill. So every 3 days it's 75% fresh fertigated water topped off. Have it rest at a volume that you can significantly refill every 3-4 days.. maybe 5? depends on a lot of local variables. If not doing so, keep the tank that holds majority of water dark and cool, preferably not in the tent where it might get to 75-80F with lights on. Getting a better brand of 'hydro' fertilzier may be the easiest, though. Any soilless/hydro nutrient system that can't maintain its pH is poorly made.

What water do you use? And what stage of flowering?

I was under the impression you want a lower PH in the res around 5.8 Can alwats invest in a montioring system that also injects PH down/up into it to keep it stable.

You're really never gonna have constant pH in a reservoir where plants are feeding, at least in my experience. Plants uptake nutrients and water in different ratios, at different stages of growth. In young plants it's not really an issue as they're not feeding much so numbers stay pretty stable but once plants really start feeding I find myself topping off my tanks pretty well every 2-3 days. I manage my tanks during these top offs. My tap water runs about 6.8 so fixing low pH is pretty simple, just don't use as much or any down next top off.