A basic problem with mixing things into the water to get the KH, GH and pH we want is that we are then forever stuck with trying to match those parameters when we do water changes. It is far better to use the tap water as it is, if possible. Of course that isn't always possible, but why introduce this problem if it is possible?
I think saying that tap water is far better is a bit of an over-generalization, not everyone's tap water may be suitable; not everyone's personal predilection may not be to accept tap water as it comes out.
I'm also not sure what you mean by "trying to match... when we do water changes". If you mean "it's too hard to mix water the same way each time"... well, not for someone who can use a teaspoon and measure volume of water.
If you mean that the tank is different at the time of water change, and we need to adjust the input water to somehow match it... well, if true, that would be true for tap as well. My plan for RODI water is to simply mix it the same way each time, and use it. If the tank drifts (let's say it becomes a bit more acidic and consumes KH over time), then when I do water changes its PH will raise a bit each time, and reach some steady state of down/up. That's exactly the same thing that will happen with tap water.
My questions here are not so much about making it the same each time, but making it match a specific value, and why it is not. It's understanding the chemistry. I could do just the same as it sounds like you do with tap, and just use whatever I get once I mix it.
I do freely admit that there is no good reason to try for a specific value in what I am doing. But you know what -- I also try to park in the middle of a parking space, if I'm shooting basketball I try not to have the ball touch the rim, only net, and if I'm docking a boat I want it to just barely kiss the dock, not bump into it, even if that's what fenders are for.
After all... if I just wanted to see any old fish in a tank I could get a screen saver.
My thoughts were identical with your last statement. I wonder if your test kit is in full calibration. We can only be as accurate as the instruments we are using.
Yeah... I can see that it is fairly precise (i.e. consistent), but especially for ph I do not have anything good to calibrate against for accuracy. I keep thinking about getting one of those handheld PH meters, or maybe a sensor to hook to the controller I have doing lights, but I worry that may just be yet another different number and then I need a tie breaker.
The thing that bothers me is the interaction with Equilibrium. Without it, KH and PH seem to track (in theory you should be able to measure one and know the other, right?) It's equilibrium that seems to be throwing me off - it is changing the PH I measure without changing the KH I measure (I say it purposely that way -- maybe whatever the PH test is, is effected by fresh Equilibrium in solution).
I'm putting the RODI away now, next time I get it out, maybe I'll do all the tests again from scratch, see if I get the same results again. And maybe pick up a dissolved CO2 test if they are cheap -- maybe I'm off on aeration and assuming about 3ppm co2. Heck, maybe Equilibrium somehow drives out CO2? Consumes it? I also admit I did not leave things for many hours to be sure I am in equilibrium (small "e") -- maybe it takes time for some of these things to settle down.
Just hoping for a chemist to stop by.. ...