So I broke out the rest of my chemistry set and did an analysis.

Brita's special filtration resin removes calcium and magnesium ions and replaces them with H+ hydrogen ions, resulting in the acidic pH of filtered water. It does not remove carbonates, which are now present in excess as HCO3-. If I were to boil this, my pH would go from 5.8 to a likely ~10 because the CO2 gas would split, leaving OH- ions in place which would cause the rise in pH.
khyberkitsune Reviewed by khyberkitsune on . Dramatic pH drop in filtered H2O? I picked up a fresh Brita water filter yesterday, and have been using it to do some minor water treatment. I noticed something - the pH drops from about 8.5 to a near-perfect ~5.8 just going through the filter. Does anybody else experience this? I typically just use straight tap but I'm willing to roll with a water change, and at least filter it before adding nutes. Looks like if this keeps doing what it does, all I have to do is add nutes after filtering, no pH down required! Rating: 5