Carbon Dioxide(CO₂) is a gas which is highly soluble in water in relation to the other two important gases found in H₂O, specifically oxygen and nitrogen. Dissolved carbon dioxide and oxygen levels in water are very important since they directly affect water chemistry (pH, KH) and are involved in essential biological processes, such as photosynthesis, in plants.

There is a gas exchange which takes place at the surface of the water between carbon dioxide (CO₂) and oxygen. The exchange results in carbon dioxide (CO₂) leaving the water and oxygen entering due to the fact that there is more oxygen in air than water and vice versa for carbon dioxide. Agitating the water surface accelerates this effect.

The significance for planted aquariums is that surface agitation should allow just enough oxygen for fish (demonstrated by comfortable behavior and respiration) and not so much that it drives out all available carbon dioxide for plants.

Planted aquariums will often benefit from the addition of carbon dioxide. Concentrations of dissolved carbon dioxide are linked to pH and KH values, with the ideal pH range being 6.8 to 7.0, and a carbonate hardness (KH) value of 90mg/L (5dKH). When adding carbon dioxide to an aquarium, it is advisable to maintain a KH value of 90mg/L and allow the addition of CO₂ to slowly lower the pH value to the ideal range. Different species of fish may react differently with respect to dissolved carbon dioxide levels, so it is advisable to monitor their reactions and take appropriate action with respect to its administration.