Why do plants need co2

All plants, terrestrial, epiphytes, and aquatics require CO2 to conduct photosynthesis, (basically a chemical reaction triggered by sunlight which produces high energy sugars). Without CO2 and water, plants cannot produce these high energy sugars... So, basically, high light plants need a lot of sugar, therefore they need to take up a lot of CO2. Without a lot of light and high energy sugars, these plants will not grow well... in fact, most will slowly die away.

If I've made a mistake or forgot something.. please let me know ...... (It's been a little while since Bio & Lab )

Andy16,

Plants require carbon because a plant, by mass, is 41% carbon. More mass (new growth) requires more carbon.

Lower lighted aquariums have a lesser requirement for carbon since it is being used (demanded) at a slower rate. Plants can utilize the carbonate and bicarbonate ions from the water column without jepordizing the KH to the point of instability.

Is it necessary for a 20g? It depends. If you are going to have a lot of lush, fast growing plants, then I would say yes, definately. If you are going to have a few anubias and java moss, then no.

It is rare that fish will produce enough C02 to supply the demands of a planted aquarium.

HTH

PP

Only a few plants can use the carbon from bicarbonates (biogenic decalcification) - Val being one of them. The majority cannot utilize that carbon source.

And biogenic decalcification can be a pH disaster very easily - the pH tends to skyrocket, as hydroxide ion is a byproduct.