Why CO2?
Plants build their tissue from carbon dioxide. There is naturally little of it in water, so under strong light CO2 becomes the factor that limits growth. Added CO2 speeds up growth, deepens the colour of red plants, and is needed for dense carpets and more demanding species.
If you mainly keep undemanding plants such as Anubias, Cryptocoryne and mosses under a standard light, you can do without CO2.
Cylinder or bio system
In a cylinder system CO2 comes from a pressurised cylinder through a regulator that keeps the flow steady. It is precise and cheaper in the long run. A compact kit such as the Tropica CO2 System Nano with a disposable 95 g cylinder suits small tanks; larger ones need a regulator with a refillable cylinder.
A bio system produces CO2 by fermentation. It is simple and cheap to start, but the amount of CO2 is less even and the refill has to be replaced regularly.
Diffuser and bubble counter
A diffuser breaks CO2 into fine bubbles so the gas dissolves in the water instead of rising to the surface. A bubble counter shows how much CO2 is flowing, and a non-return valve keeps water out of the tubing. The Tropica 3-in-1 diffuser combines all three.
How much CO2?
Start at about one bubble per second and increase gradually over several days. Check the level with a drop checker: green means about right, blue too little and yellow too much. Watch the fish: if they gasp near the surface, reduce CO2 at once.
Switch CO2 on about an hour before the light and off about an hour before the light goes out: plants do not use CO2 in the dark. A solenoid valve and a timer make this automatic.
Do not forget fertiliser
With CO2 plants grow faster and need more nutrients too. Tropica Specialised Nutrition is made for densely planted aquariums with CO2.
