CO2 is generally (but not exclusively) used by people with "high-tech" plant tanks. In other words, people who have high amounts of lighting (usually around 2 watts/gallon of normal flourescent lighting, or more) are the ones who usually use CO2 injection. However, it is my belief that even "low-tech" (under 2 watts/gallon of normal flourescent lighting) can benefit from CO2 injection.
There are 4 major ways to introduce CO2 into a tank:
1.) Injection via. CO2 production from yeast. CO2 is generated in an air-tight container using a yeast/sugar mixture. As the yeast metabolizes the sugar, it produces CO2 as a by-product of anaerobic resipiration. As CO2 builds up, it creates enough pressure so that CO2 can move into the aquarium (via. an airline attached to this container). This is the simplest and cheapest (on start up) method of CO2 generation, however it generally only works on smaller tanks. Tanks under 20-30 gallons will do well on such a system... but any tank larger will need multiple yeast+sugar containers.
2.) Injection via chemical dosing. Seachem has a product called Flourish Excel. This product introduces organic carbons into the tank. It also works well, but only in smaller tanks (20-30 gallons and under). Larger tanks require more dosage, which will become uneconomical for the user. This is also a simple way of introducing CO2 into an aquarium, but it is also perhaps the most expensive way.
3.) The
Carbo-Plus system is also another way of introducing CO2 into an aquarium. However, you will need to replace the carbon bar every now and then... it can become rather expensive as well. Effectiveness of this system can also depend on the hardness of your water.
4.) Pressurized CO2 injection is perhaps the most effective way of introducing CO2 into an aquarium. This method involves using a CO2 tank, a regulator, and a needle valve. Though it is the most expensive thing to use at startup, in the long run, it is the cheapest. I refill my lbs CO2 tank once every 6 months at a cost of 6 dollars CDN. It is one of the easiest methods to control. Many experienced plant users use a pressurized system.
All CO2 units will need a reactor of some sort. This reactor helps faciliate the dissolving of CO2 into the water columne. Reactors can take up an entire other thread.... so I won't comment much about them. If you have questions about a reactor, please don't hesitate to ask.
The unit that you've linked, is essentially a commercialized version of method #1. Its results can be easily duplicated by a little bit of DIY work. If you're interested in trying your hand at making your own CO2 generator, ask, and/or look around this forum (look up "DIY CO2"). There are a lot of plans out on the internet on how to make your own CO2 generator.
HTH
-Richer
