For any of you (flattrack) who are interested in building your own CO2 generator, here is how it is done. Very, very easy and super cheap!
You will need:
1x 2liter Soda Bottle, emptied
1tsp Baking Powder
Fresh Warm Water (not from the aquarium)
CO2 Proof Tubing
Small amount of silicone sealant
1) Drink the soda, preferably in one sitting.
2) Drill a hole in the soda cap large enough so the tubing will fit snuggly.
3) Push the tubing through the soda cap so it sticks out maybe 1/2 inch through the bottom of the cap.
4) Seal around the tube on the top and bottom of the soda cap, allow it to dry and make sure that it is air tight!
Once the silicone is dry and it is air tight, it's time to create some CO2!
1) Add the sugar to the bottle (1 cup).
2) Add the yeast and baking powder (1 tsp each) on top of the sugar.
3) Add warm water (1 liter) to the bottle, leave the top 4-5 inches empty as this room is needed to produce the CO2.
4) Cap the bottle and put the other end of the tubing into the intake of your canister filter (in the tank).
The CO2 should start being produced in a day or so, you may notice the occasional bubble being released into the intake of your canister filter. The CO2 will get all mixed up inside the filter and will dissolve into the water. If you were to just place the tube in the tank and let it bubble, the CO2 would leave the tube and float right to the top of the aquarium and would be pointless.
As far as the measurements go, you can figure out what works best for you. The sugar is like food for the CO2 generation so the more sugar you use, the longer the mixture will last. The yeast is what actually reacts and creates the CO2 (along with the water and sugar of course). And the baking powder is used to help stabilize the CO2 production. The amounts stated above should last about 2 weeks.
One 2 liter should be good to supply CO2 to a tank up to about 50 gallons. For larger tanks, just build more CO2 generators!
By the way...
You can also just plug the tubing into an airstone and let it mix with the water that way (but the canister filter method will probably work better).
The CO2 may change the PH of your water.
Once you have started using CO2, don't stop as this will again change the PH of the water. Also, do not close or kink the tube as this will cause excessive pressure to build up in the CO2 canister and it may explode. Although it would look/sound cool, it would suck to have to clean it up.
Also make sure that you have good lighting as it is needed for your plants to grow!