> There's no reason to top up if the wine is still fermenting.
> The CO2 produced during fermentation will drive out any residual
> oxygen and thus oxidation is not a concern. Once fermentation
> has stopped then no CO2 and topping up is important.

Even when fermentation has stopped you don't need to immediately
top up. No need to worry about that until you've opened the carboy
as fermentation has replaced the O2 with CO2.

Therefore at the end, until you need to open it to say rack it
off the lees, you can just leave the wine sitting in the carboy
for another couple of weeks.

Don