The 1st thing you need to know to understand stability or lack therof is the chemistry of your make-up water. My make up water comes out of the tap above 7.8 and that's part of the reason I add acid on a regular basis. My pebble sheen is still less than 2 years old, so it may still be driving the PH up as well. I use about 2 gallons of acid a month in a 14,000 IG pool