Hi Ray, I use tap water. The characteristics are: pH = 8.1, EC = 0.25 mS/cm. Yes I start with a pH of 8.1 before adding the nutrient. These measurement are very consistent with time. After adding the nutrient, at 30 PPM of nitrogen, I get pH = 7.1, EC = 0.45 mS/cm. This what I start with in the tray where the pots sit. After 10 days this becomes: pH = 7.9, EC = 0.80 mS/cm.

About 50% of the water I drink is carbonated - we carbonate it ourselves using tap water (we use a Sodastream). After carbonation the water characteristics are: pH = 5.3, EC = 0.25 mS/cm. The carbonation lowers the pH significantly but does not affect the EC.

What may be happening is both evaporation, which concentrates the solution and hence raises the EC, as well as carbon dioxide absorption which lowers the pH.

Two things:

Don't just consider pH of the water, look at the alkalinity, as well. Alkalinity is a form of buffering, in this case, resisting the effect of an acid to lower the pH.

I also think you're missing the action of the plant, which is not a simple uptake situation.

As a plant absorbs a nutrient ion, it must secrete a compensatory ion to maintain neutrality. If it absorbs a nitrate ion, with is negatively charged, it must secrete a negatively charged ion into the rhizosphere (including the solution in it), which raises the pH. If it absorbs an ammonium ion (+), it secretes a "plus" ion, lowering the rhizoshpere pH.

That is why I reformulated K-Lite (2% ammonium, 98% nitrate) to K-Lite Plus (15% ammonium, 85% nitrate), to give more balance.