I have been making wine for 35 years and never topped up my carboys., nor have I had any problems. trick is to keep thoolsrilized and airlocks full, I use a solution of 3 grams k-meta and 11 grams tartaric in one gallon of water in a spray bottle. I spray anything that will touch the wine. I also keep the so2 levels up in my wines. the amount of time that the carboy or other vessel is open to air is not sufficient to oxidize the wine. as so2 is a oxygen scavenger as well as a bacterial inhibitor proper so2 levels protect the wine.
