Spidey wrote:No worries. The CO2 from the fermentation will pump most of the ambient air out of the carboy. Then, since CO2 is heavier than air, the CO2 will protect the mead from oxidation if there happens to be any air left in the carboy. 1 gallon of headspace won't be a problem.
+1. As a note, all conicals (professional grade included) are designed to have 10-20% head space when filled to the rated capacity. I've even seen 15bbl batches go into 30bbl fermenters without issue.
