http://www.plantedtank.net/co2.html

a CO2 generator you can build yourself to give your plants plenty of CO2 for photosynthesis

BEWARE CO2 LOWERS WATER PH SO THIS IS AN EXPERTS ONLY THING DONT DO THIS IF YOUR A NEWBIE UR FISH MAY DIE

That looks a lot like my CO2 reactor! It works great! A word of caution... DO NOT USE AIRSTONE TO DIFUSSE CO2! It will get clogged eventually and the pressure will build up in the bottle to the point that it will burst and make a HUGE mess.

Also, do not use the filter as the difussion mechanism because you will loose a lot of the CO2. First, agitation by the impeller and the pouring of the water into the tank will allow CO2 to escape to the atmosphere. Second, sertain organisms in the biofilters use CO2 as their carbon source.