To control algae, have faster growth of plants and overall wellness of their tanks, co2 can help with these desires.  Most people on this site that use co2 do the yeast and sugar  co2 reactor method.  Over time it gets annoying to deal with making these reactors and they're not exactly a stable method of consistant co2 injection.  Also you cant "turn off" the reactor.  Instead of using a co2 reactor try the paintball co2 injection method

http://www.plantedtank.net/forums/showthread.php?t=115850

At the site above, any regular paintball CO2 canister can be used to inject co2 into your planted tank.  The price to make your pressured co2 system can range from 40 US dollars and more depending on if you want gauges (recommended) solenoid regulators to turn off the co2 at night, diffusers, aesthetics, and so on.  The prices listed on the website above are not correct because of the costs of items now, so do not rely on those approximations.

Just a little info on CO2 - while CO2 is usually the limiting factor in all low tech planted tanks and people want to rectify that by injecting CO2, do understand CO2 will bring its own complications.  If you are NOT careful you can cause your tanks water chemistry to become unbalanced and you can potentially kill all your fish, and possibly your plants.

• CO2 is a corrosive gas.  If you get the bright idea that "i can use any tubing I have here" you are wrong. you need CO2 resistant tubing, usually made out of silicone.  
• Your regular air line tubing will NOT work.  The rubber will be eaten away by the CO2 acidic gas.
•  Also you must make sure to use check valves on the CO2 tubing to prevent water from backing up into the tubing.
• You must have enough buffer (alkalinity) otherwise the CO2 will cause your pH to become more acidic, even too acidic and cause fish death and possibly metal toxicity if left unchecked (fish dont like changes in pH more than .5 in general)
• If you leave your co2 injection on throughout the night when your tank lights are off, you can cause a huge change in pH if you dont have nearly enough buffer.  Dead fish are not happy fish are not happy you. Its best to turn off the co2 at night (hence the desire for a solenoid regulator)
• Use a bubble counter to see what your co2 diffusion rate is.
• For diffusing your co2 do NOT use an air stone, they're VERY inefficient. you can order a cheap ceramic difusser online.  they're easy to clean and it will do a better job at NOT wasting your CO2
• You should check your co2 levels via a drop checker or by using the explanation and chat on Aquascapes.info page
• For an tank that has fish and planted, do not go pass 40ppm (some say even 35ppm)

Remember pressurized canisters holding liquid CO2 is NOT  a toy so respect the physics thats holding your canister together. Exploding canisters are not fun, letting the co2 out very quickly will cause the canister to become FREEZING to the touch. you dont want your hand frozen to the tank, You dont want to pass out due to CO2 displacing the oxygen in your room possibly killing you

If you play it safe youll be fine. Sorry for all the doom, but I just want to make sure you understand if you use co2 in your tank, you should be mindful of the responsibility of having pressure co2. 

With CO2 injection youll have happy faster growing plants

Happy tanking and trimming!!