Conical Fermenter vs Carboy

The oxygen question

Two weeks versus nine months

Oxygen is the enemy of finished beverages, but the timescale over which it does damage is long. A beer that ferments for two weeks and gets kegged has no meaningful exposure to slow permeation through a vessel wall -- it is gone before that matters. A mead that sits in bulk for nine months absolutely does.

Glass is the only completely oxygen-impermeable common option, which is why wine and mead makers use carboys and demijohns and beer brewers mostly do not. PET is slightly permeable, which is irrelevant over weeks and relevant over seasons. Stainless is effectively impermeable but a conical is usually the wrong shape for aging, because a part-full conical has a large flat liquid surface in contact with headspace.

That shape point is the one most people miss, and it is why the real wine answer is neither of these -- a variable-capacity stainless tank has a floating lid that drops to the liquid line, eliminating headspace entirely.

The glass warning

Do not use glass carboys for beer

Glass carboys are still sold as the premium beer fermenter and they should not be. The problems are practical and they are serious:

• Weight. A full 6 gallon glass carboy weighs around 60 pounds with no handle and a slippery wet surface.
• Breakage. Glass fails without warning under thermal shock or impact, and a full carboy breaking is a serious laceration risk. Milk-crate carriers exist precisely because this happens.
• Cleaning. A narrow neck means a brush and a lot of hope. You cannot inspect the inside properly or reach a krausen ring with a cloth.

For beer, a wide-mouth PET carboy gives you the same shape with a third of the weight and no breakage risk, and the slight oxygen permeability is irrelevant on a two-week timescale.