CO2 is fun to experiment with.
I have used CO2. The cheapest method is mixing a packet of yeast and 6 cups of sugar in a coke bottle. Drill a hole in the top. Then seal a 1/4inch tube through the top to a one way check valve. Then run the top into the tank. This will bring a 10gallon pH to about 6.5 from Detroit city water. The pH will drop to 6 or below, if the CO2 hose goes directly into the external canister filter. The pump on the filter will crush the CO2 and lower the pH more. However, try not to drop the pH below 7 for the health of the fish.
Another way is to use a CO2 tank. I have tried a few different systems. I have concluded that CO2 is fun to experiment with, but I do not think it makes the plants grow faster or healthier.
After talking with plant specialist, substrate and lighting seem to be more important. River muck is the substrate that the specialist are using. Then sand is put over the river mock. I think river muck is great, but I do not like the appearance of it. So, I use iron laterite from amazon.
http://www.amazon.com/gp/product/B0002566U6/ref=oh_details_o00_s00_i01?ie=UTF8&psc=1I mix a handful of laterite per 10 gallons with fine sand. I get the fine sand at home depot for $3.00 for 50 lbs. I'll post pictures of a 10 gallon I experiment, if you'd like to see it. Long story short, CO2 is dangerous and it did not show any considerable difference in plant growth. In my opinion it is not worth losing the fish, when there is no noticeable difference aside from the pH.
