Just so you know, making your own co2 is fine and all for 1 - 4 plants... but if you plan on going bigger, for around 55$ u can get a 5 lb co2 tank already filled, then all u need is a regulator(25$) and a co2 bubble counter(5$) On ebay they do sell electric cylinoid co2 regulators with a bubble counter built in(65$) so that you can hook it up to a timer. All this for around or under 100$.

Go check your local beer brewery shops, they have more co2 than they can sell and its cheaper than making it yourself.

For those who want more, pick up a 20lb tank for only 150$ - 185$ and it should only be around or under 1$ per lb of co2 you buy.

When using a co2 tank you can do really cool stuff like throwing a co2 reactor into your hydro system, and if you are using a drip system or hydro buckets, you can fuse the co2 lines into your air lines and mix co2 into the water as well as distro through the drip sytem. Growing can be alot of fun if you keep it simple.