I add Seachem Reef Builder (alkalinity buffer)... I don't buffer the top-off water with the intention of keeping the pH of it or the tank at 8.3. My intention is to use it to help maintain the KH of the tank between 8 - 9 dKH. Our 90g evaporates 1 - 1.5 gallons / day (varies depending on how much the heat or AC runs) and I add 1 - 1.5 teaspoons of alkalinity buffer / gallon of fw in the top-off reservoir. Given the rate of consumption in our tank, that amount works well.