I got some great apple juice from New England from my local place last year; I believe I got six gallons and I fermented it first in a seven-gallon bucket and then in a six-gallon carboy. I used sweet cider yeast (my husband likes it sweet), and it came out great. I used the homebrew place's recipe, but it is not online anymore, so I am recreating it from my notes. I cannot find my notebook right now, but I remember adding apple juice concentrate. I think it was to top it off, but I'm not sure. This year I want to make more than double the amount and make it in two "six-gallon" (really 7.9 gallon) FastFermentors. I ordered 12 gallons of juice, but wasn't thinking of the airspace. There is a much larger airspace in a FastFermentor. Is it going to be mostly CO2, or do I need to order more juice to fill each one up?