Heres part of an article about succesfuly growing plants in aquarium.
What do u think, good idea?
CO2 dissoves into (and escapes out of) water very quickly, so we need a way to produce bubbles of CO2 and to hold them in contact with a fast flowing stream of water so the CO2 has time to dissolve. CO2 is produced by yeast fermenting sugar into alcohol, so take a 2-liter soda bottle and fill it with lukewarm water to about 2" from the bottom of where the screw cap would be. Pour the measured water into a bucket and add approximately 2 cups sugar and 1/4 teaspoon baking yeast (e.g. Fleishmann's brand from the baking section of Safeway). Stir until both are dissolved, especially the yeast which is harder to dissolve than the sugar. Pour this stuff back in the bottle and fill to the point it normally would be filled with soda. Drill a hole in the center of the top of the cap which is just wide enough to tightly fit a piece of aquarium airline tubing into it, and glue the tubing into place with aquarium silicone sealant. Leave the cap off the bottle to dry for a day. Then screw on the cap and put the other end of the air tube into the intake tube of the filter, so that the CO2 will bubble into the filter. The CO2 may start bubbling the next day, or maybe not for up to 3 days. The bubbles get sucked into the pump propeller and some end up in the filter sponge where they slowly dissolve into the water where the plants can use it for photosynthesis.