Mark,
The loss of CO2 though the wall of the tubing is really not that great, certainly not of concern for the 10 inches of so of line of flexible tubing. I use standard Homedepot 1/4" rigid tubing for running CO2 all over my house to my three calcium reactors. Only the last 10" or so is flexible tubing. You don't really need, IMO/IME CO2-stable airline tubing, CO2 is not really very likely to degrade tubing. Pure O2 would do a lot more damage.
FWIW, when I get my 20# CO2 cylinder refilled it costs $10. If you are going though CO2 fast I'd look for a leak in your system and not assume that it is leaking THROUGH the tubing at any significant rate.
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