CO2 -- i made my system for like a buck. basically there are 2 parts to a CO2 system: generating the CO2, and getting it into the water. to generate CO2, ive got a 3 liter (2 liter is fine) bottle with a hole in the cap and a hose (seal that hose into the cap with any aquarium safe sealant) going from the hole to the venturi intake on a fluval 2+. in the bottle i mix 3 cups sugar (1 per liter), 1.5 teaspoons of yeast (.5 per liter) and enough water to fill it to about 4 inches from the top. shake well, and CO2 production should start in an hour or so depending on water temp. the only hardware in my tank is the fluval 2+ (basically a power head with a "corner filter") so i use that for my infusion. my planted tank is only 10 gallons, so this gives me enough. if you're running a larger tank, you will probably need a better infuser. ive seen several DIY setups involving a power head, a gravel vac, and some super chunky substrate or bio balls or filter wadding. you cut off most of the hose of the vac, leaving just enough to connect to the power head. drill a hole into the side of the vac halfway down, and attach the other end of the hose (with the bottle cap being on the other end). put enough chunky substrate or bioballs into the vac so that it comes up just under the side hole you drilled, and plug the bottem with filter wadding (sometimes called "floss") and attach the vac to the inside wall of your tank with suction cup mounting brackets (a couple more bucks) next to the power head.
so here's how it works: the power head constantly forces water down into the vac, pushing it thru the bio balls and floss (free bio filtration
). when the CO2 comes in through the side hose, it wants to rise against the current from the power head. the bubbles generally cant get through the filter wadding, so they kick around in the vac. in the process, it gets all churned around by the current and bioballs, and dissolves into the water. voila.
total cost assuming you need to buy everything listed is maybe 30 bucks.