Actually, you can't measure the pH of good RO water. In RO water there should be very few dissolved solutes (2-3ppm or less) and thus you could get pH readings of anywhere from 6-9 for RO water depending on the sample you take. Adding salt mix to RO water contains all the solutes that raise the pH. Salt mix is more than just salt, there's calcium, carbonate, bicarbonate, stronium, iodine, magnesium, and others in there that all help bring the pH up. Seeing pH swings within the first couple days of a tank's life is normal, as is lower pH for new tanks once things settle down.
