Carbon dioxide (chemical formula: CO₂) is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state.
CO₂ is used by all plants during photosynthesis. This is to make sugars which may either be consumed again in respiration or used as the raw material for plant growth.

CO₂ in the aquaurium
When maintaining a planted aquarium it is very important to keep the CO₂ at the appropriate levels (see chart below). More often then not, this is accomplished by CO₂ injection.

CO₂ levels chart

Other information on CO2 and Photosynthesis

• DIY CO2
• CO2 Systems
• Oxygen Tanks
• Fire Warning
• Compressed Gas Safety