Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
Common clear airline tubing is vinyl. Over many years, it gets brittle, and can crack and leak. This accelerates where both CO2 and water or moist air meet, because they combine to form carbonic acid, which attacks the vinyl. If you use this, you may find it needs replacement every year or so. Maybe more, maybe less, all depends on the specifics of the setup.
Many LFS also sell silicone tubing, which is typically a translucent blue. This doesn't get brittle or need replacement, but leaks 55x more CO2 than vinyl tubing. What gets left out of this oft-repeated fact, is that 55x an incredibly small amount is still a very small amount. So small the loss is inconsequential unless you're running a line clear across your home. I recall an experiment where someone attempted to put the leakage to use as as a diffuser, by putting a whole roll of it in their tank; and it didn't leak enough CO2 to make a measurable change in the pH. I typically use this for DIY CO2, because it's relatively cheap, and readily available. I don't recommend it for pressurized, since it's a little too flexible; under the pressures involved it can easily pop off fittings, and might even balloon.
Some folks claim the exact opposite, that silicone gets brittle and vinyl does not, when exposed to CO2. Or even that both get brittle. It's possible some have had these results. But they don't match my own results, or the majority of reports I've seen.
CO2 resistant tubing can be any number of things, but the tubing I received from two different sellers appears to be flexible PVC. Although this is polyvinyl chloride, it's not the same as plain vinyl, and won't get brittle. I hear polyethylene works too, it's just rather stiff by comparison. But again, there's all sorts of conflicting info out there. So for pressurized systems, I just order CO2 resistant tubing from reputable dealers. Not a place to cut corners or deal with uncertainties, in my opinion.