Honestly, I'm not trying to argue, but I've never noticed them to make a noticeable impact on my PPM's. I've measured several times, but it's never been enough to actually register... (for me). I know in theory this is true, but it's never been measureable when I've checked it. Might be a different type of PH up/down?
There are different opinions on adding nutrients to increase your PPM between res changes. I am not for it, personally. What I do and advocate is to do weekly reservoir changes, and in between if reservoir levels need adjusting, they should be adjusted with just water. Other than that, it's just adjusting the PH throughout the week until the next res. change.
Now, some GOOD growers top off with nutrient water instead of just water when they need to add to their reservoir between res changes. But, I don't personally advocate this. A lot of people, like myself just top off with plain water when needed.
Plants will cherry pick nutrients. Lets say, your PPMs go down by 300ppm (just for hypthetical purposes). What's missing - what did they take? The plants take what they need, and WILL cherry pick. So, lets say that they took primarily phosphorous. So, now you want to raise the PPMS? If you add nutrient water, you are giving it more of
everything, not just the phosphorous it took. This is an exagerrated example, but nonetheless relevant.
In DWC hydro, plants get an abundance of what they need at all times - they're constantly exposed to the buffet table. They also store energy they need in the fan leaves. Letting the PPM's go down a little between weekly reservoir changes will not hold you back as far as plant/bud size. Conversely, you run some risks adding nutrient water to "top off" between reservoir changes.
I prefer to just adjust the PH between weekly reservoir changes, and top off with RO water if the levels get low in the reservoir. I don't try to manage to that PPM number beyond making sure it doesn't go up, (due to plants drinking more water), and being aware of it. I'm okay if it goes down a little, though it might tempt me to feed a stronger mix at the next reservoir change.
Sorry if I rambled, but I wanted to make sure I was explaining my process and logic correctly. Hope you don't mind the long diatribe, A1.