This breathable silicone stopper fits 3, 5, 6 , and 6.5 gallon glass carboys and 500 ml flasks. This will not fill PET carboys or jugs.
A breathable Silicone stopper acts as both a stopper and an airlock. Its unique design allows CO2 to escape while keeping oxygen and airborne contamination out. It will also create a vacuum if the fermenting liquid cools (as opposed to pulling water in).
Great choice for fermenting wine, lagers, or anything with an extended fermentation time.