feed your working reservoir via a float valve connected to an elevated mixing /volume tank and your add back and topping off days are over.
with experience you can steer the working part of the solution from the mixing tank by varying the nutrient amounts and ph adjustment.
for example, if i see 6.4-6.5 at 1250-1300 i will run a load of water only ph'ed to about 5.2 into the working part from the tank.
because it is being fed by a float valve the changes in the working part occur slowly and gently. it will have a very small immediate effect but over the course of a few days will correct to acceptable levels.
what you input and how much to correct is dependent upon your water supply, nutrient choice, uptake rates, environment, etc.
this allows me to go through an entire grow without doing a mass change out.
float valve!
Good idea but you missed an element of my design
I have no working res ... These are self contained . A 5 gallon res under each pot . No drain lines . No central res . Each PPK has its own pump .