What are the benifits of having Co2 in a tank for fish and plants?
Almost everything you need to know is here...
http://www.csd.net/~cgadd/aqua/articles.htm
I think you will find the information at the link Darkphreak posted very useful.
The short answer is that CO2 is one of the nutrients which plants require. Some CO2 is naturally present in water but in a tank with medium to high lighting plants will grow quickly and use up all the CO2. To keep the plants growing properly it's necessary to add more CO2. "Standard" aquarium lighting is considered low lighting; adding CO2 is not necessary in a tank with standard lighting.
Adding CO2 is not required for fish's well-being.
Marc
General info also can be found here:
http://www.cichlid-forum.com/articles/carbon_dioxide_planted_tank.php
ok thanks