Thanks for the info, Rex. I did see the thread about the permeability of silicone and the possibility for escape of CO2 in silicone tubing, and while I don't doubt the math in a theoretical situation, I was thinking that the gas would likely follow the path of least resistance and go out the large opening at the end of the tube rather than go through the tubing itself, given that it isn't under tremendous pressure in a DIY system while in the tube itself (since the pressure is released in the bubbles at the end of the tube regularly). In a closed system where the tubing had no large outlet at the end, I would think that the CO2 loss would be significant, but I figured I'd experiment and see if that's the case in the real world condition of the tank.
I made the switch because I was looking for something a little more flexible and I'm seeing the same bubble rate with the silicone that I saw with the clear airline tubing (counted immediately before and immediately after the change), but if I see a drop off, I'll switch it back for sure. I'm just experimenting to see what happens with this tubing.
If I do see the kind of CO2 loss that the calculations would predict, though, what would you suggest as the best alternative? I used the black "stealth" airline tubing in the past and liked it because of its looks against my black background and its flexibility, but if it's no better than the silicone in terms of permeability or no better than the easier-to-find clear stuff in terms of remaining flexible and intact over time, then I'll look for another option. Thanks for any suggestions.