buffering topup water

Yes, I do. I add alkalinity buffer to our top-off reservoir and use the auto top-off to keep the alkalinity of our tank in the proper range.

Be sure not to overdose... The last thing you'd want is too much going in and throwing off the overall water chemistry balance in your tank.
 
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.
 
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