I keep having a problem with stale paper towel tasting beer. I'm sure it's a oxygen problem. I usually ferment in a 6.5gallon carboy and fill it to about 4 or 4.5gallons of wort. I top off with burst of co2 and am always carful when using my auto siphon into a keg. Is the 6.5gal carboy to big for a 4gal batch?