Hmm. I keep forgetting that you are not using this for maintenance purposes but to kill the invaders. (By "bad stuff in my tap water," I'm assuming that you mean bad stuff in your system and not actually in your municipal water-supply?
)
Considering that, what I'd do if it were me playing in your attic would be to just hose everything possible with H2O2 (not straight at ~30%, although I could not speak on the likelihood of incurring plant fatalities in doing so; I just don't feel that that strength would be required by a fair margin), rinse everything off, then mix up a fairly strong H2O-H2O2 solution without bothering with pH-adjusting - since you are chiefly concerned with entities external to the plant - and run it for up to a half-hour, then drain, rinse, and refill as per normal and add your H2O2 as a supplement. If you didn't get all the crap killed/rinsed you might still see some pH-shifting but you might as well balance it soon after doing up the reservoir. A half-hour afterwards is not too long to wait though if you
expect further shifting. The plants will not suffer any real harm in the meantime (they might wilt a bit if you add silicone and it spikes but they would (should) bounce back after straightening.
IDK. My med schedule has gotten messed-up over the past few days and I'm hitting the nighttime mental low in the cycle. I'm blaming the pshrinks. So... Do what appears to make the most sense to you. Don't go away for a day afterwards to ensure that if you have to adjust or even replace, you will be able to do so. You're smart and attentive enough and your plants should come through with less stress than the grower will encounter
.