Would it be harmful to ferment say a 7 Gal batch in a 15Gal Primary fermenter? The way I see it, the amount of surface area exposed to oxygen is the same, no matter how full the fermenter is, but would the amount of head space in a half filled keg be harmful to the fermentation process, or produce undesirable results? Trying to decide if I want to build a fermenter out of a 1/2 barrel keg, or use multiple corny fermenters.