i was woundering how the co2 affects the fiish ? and how important is it to the plant...i have a 55g with 5 plants and 10 fish, do i really need to and co2 if i want my plants to live...and if so can i kill my fish with to much?
Basically, CO2 is usually the limiting factor for plant growth in most tanks.
CO2 doesn't displace O2, so it doesn't directly affect the fish.
With 5 plants (would be helpful to know what species they were) in a 55g, you more than likely don't have near enough plant mass to necessitate CO2 injection.