Rather than buying buckets or carboys, I would like to use 5 gallon PET water bottles that I can get free from work (I verified they are PET with the "1" on the bottom). However all my extract recipes are for 5 gallon batches, which would not leave any room for fermentation.
Is there any harm in reducing the volume to about 4 gallons for fermentation in these containers? I like higher gravity beers, for what it's worth, but I don't know if there's an upper limit. My next batch is going to be a Russian Imperial Stout (OG: 1.082).