Is it possible to have to much headspace during primary fermentation? I am going to do some experiment all grain test batches like 2-2.5 gallons or so. Except the smallest carboy I have is 5gal. and I don't want to go out and buy another. So would 2.5gal. of headspace cause possible problems with say oxygenation? It seems very unlikely to me, I just don't want to brew a whole batch only for it to go bad from such a silly thing like this.