Day 7/21/30 - Top up, Water vs. Wine
The biggest enemy of wine is oxygen. If u bulk age, make sure your carboy is "completely filled up", leaving ~2-3" air space below rubber bung + airlock, for possible expansion(wine) due to temperature change.|
Day 7 - top up with water per manufacturers' instruction, if any.
Day 21 (and or Day 30 if u were to bulk age your wine) - Believe the recommendation from manufacturers are mixed, some suggest water, some suggest wine. Amongst older wine makers, we all agreed to top up with wine only. Most people claim that they can taste the last water addition.
For top up, best to use same style wine. Most of us just top up with whatever wine we have on hand, too expensive to top up with commercial wine. The little addition of different wine won't affect the taste much. One of my friends religiously makes a batch of Chateauneuf de Pape for top up. The wine style is supposed to be a blend of ~13 different grapes, theoretically should blend well with most wines.