Why do the instructions recommend to top up the carboy with wine, and not water?
We recommend topping up with finished and filtered wine, in order to not dilute your newly fermented wine. For example, adding 500mL (16.9 oz.) of water to a full batch of wine with 13% ABV will bring the final alcohol level to 12.7%, as well as slightly unbalance the pH and acidity. On the other hand, 500mL of wine at 11% ABV added to a full batch of wine with 13% ABV will maintain the final alcohol level very close to 13% (12.95% to be exact), as well as maintain the acidity and other important parameters close to their original and intended values. Please note that topping up your carboy is very important in order to avoid oxygen contact that can ruin your wine!