The glass Carboy is a common primary and secondary fermentation vessel. Ranging from 1 gallons to 6.5 gallons this versatile container is impervious to oxygen making it an ideal secondary fermenter. If being used as a primary fermenter it is suggested to use a blow off tube for the most vigorous part of the fermentation. Cleaning glass carboys can be tedious, but since they cannot easily be scratched they are very sanitary. Carboys should be kept in a dark place or covered, because light will prematurely "skunk" the beer.
Plastic Water Carboys
Another type is the inexpensive plastic carboys used for office water coolers. These carboys are usually easy to find at regular retail stores and are significantly less expensive than glass carboys. However, these carboys can be made of any number of different types of plastic, and a homebrewer should identify the type and make sure it is suitable before attempting to use one as a fermenter. Looking at the recycling code on the bottom of the carboy, #1 plastic is PET and should be suitable for fermenter use. Other codes are less suitable, particularly #7 which represents "Other", and could thus be just about anything. Using a carboy made of unsuitable plastic may mean excessive oxygen permeability, and the plastic may be porous enough to allow it to stain and harbor odors and possibly infections.
These carboys are also typically only available in a 5-gallon size or smaller, making them unsuitable for use as primary fermenters for 5-gallon batches as they have insufficient headspace for krausen.