Daaamn, that tap water is kinda thick!! The last house I grew in, our tap naturally came out just 100 ppm @ 0.7 conversion, and i'do run it through a Brita and it would drop it a few more points..
I don't think Hydroguard does a lot with pH swings, but when I used to add that, I also added Cal/Mag and Silica, so the numbers bounced around a little..
Any way to use a little less of the feed you've got??
Another idea, if you have the means to do so, is make up your nutrientire mix in a separate bucket/container, with airstone (s), and bubble it for a day.. Don't mess with anything, not pH up or down, nothing.. After 24hrs, all of the buffering and chelating should be done, and you should have a better idea of what it's actually doing in your water..