I don't use CO2 injection, but here's a way to do it if you want it:
http://www.geocities.com/Heartland/H...637/index.html
You can get all of this stuff mail order. For example, here's a place selling Laterite:
http://shop.store.yahoo.com/wetpetsu...hplprla20.html
You can also mix Flourite with gravel. I can verify that Flourite works very well on its own, perhaps wouldn't work so well mixed with something else?
Or you could buy Diana Walstad's book, read it, and use soil under your gravel, assuming (1) it's small enough, and (2) it isn't plastic-coated. But if you're looking to do this for the absolute minimum amount of money, probably the Laterite would be cheaper than Diana's book!