Inactive snails, Carbon Dioxide issue?

[stevepully]

I keep quite a few different species of snail but never experienced any problem until now. I have some Achatina Iredalei and they seem to be very inactive. I keep them in a plastic tank with lots of ventalation at the top of the tank. Upon reading an article last night it made me wonder weather carbon dioxide was the cause of their inactivity, as carbon dioxide is heavier than air and the gas that snails exhale it may sink to the bottom of the tank leaving the snails with a lack of oxygen. The particular article was saying that ventilation at the bottom of the tank maybe the answer but obviously i paid quite alot for the plastic tank and would be afraid of shattering it if drilling air holes. Has anyone got any experience with this or any advice to offer (maybe a way of re-newing the air supply?!) or is it just Iredalei in general being "lazy" snails? I keep 8 other different species in smilar conditions and dont have an issue with any of them.
Here is a link to the article for further reading.
www.weichtiere.at/english/gastropoda/terrestrial/helix.html
(You may have to select the "Snails in a Terrarium" section on the left hand side.)

[coyote]

I've seen this article, and it makes sense that not enough oxygen would make the snails less active than otherwise.

Drilling holes in the plastic would be tricky. An alternative is to heat up a worn-out screwdriver or other long, thin, pointy tool over a gas flame and poke that into the plastic to melt a hole in it. (Obviously, empty the tank out first, and do this outside in the fresh air as some plastics release noxious fumes when heated in that manner.) The tool will probably get melted plastic stuck to it, so use a worn-out one for this in case it gets ruined.

Some of the harder plastics won't melt, though, so it depends on what kind of plastic your tank is made of.

Another alternative is to take the lid off and fan the inside of the tank with something such as a folded-up newspaper, magazine or even the tank lid itself, to fan fresh air into the tank. (I use the tank lid for this with my tanks.) I suppose this would have to be done several times a day. This avoids having to drill into the tank or otherwise modify it structurally in any way.

If there are a lot of live plants in the tank, theoretically they would be a source of extra oxygen for the snails, so that would be yet another means to explore. (I myself have had no luck at all keeping plants alive in my tanks.)