Carbon Dioxide (CO2) is the most important plant nutrient. Almost half of the dry matter of any plant is carbon. In natural waters with luxurious plant growth a continuous flow of CO2 is supplied by CO2-rich nutrition springs and by fermentation processes in the ground. This natural CO2 supply is missing in the aquarium and an additional CO2 fertilization is essential. A deficiency in CO2 is the reason for unnaturally high pH-values and stunted or dissolving aquarium plants.

CO2 101 - Introduction to Carbon Dioxide

For plants to grow, they form carbohydrates (sugars) out of carbon dioxide (CO2) and water, using light as the energy source. This process is called photosynthesis.

In an aquarium fish and bacteria produce CO2 continuously. While plants use CO2 and have a net production of oxygen during the day, at night this process is reversed and they use oxygen and produce CO2.

In a well-planted aquarium the amount of CO2 that is produced naturally is relatively small and therefore the CO2 concentration is usually the most dominant factor limiting plant growth.

CO2 Equilibrium

CO2 gas dissolved in water can take the form of bicarbonate or carbonate. These three forms are in chemical equilibrium. Which form is present in what amount depends on the pH.

Dissolved CO2 <=> bicarbonate ion <=> carbonate ion

In the pH range favorable to water plants, pH 6.4-7.2, a percentage will be present as dissolved CO2 and the rest as bicarbonate ions.

From pH 8.0 to pH 8.8 there will be almost no dissolved CO2, a large amount of bicarbonate and a small amount of carbonate.

Although they are named "water plants", most aquarium plants are really swamp plants that grow in their natural habitat with at least part of their leaves above the water line. Therefore, they have adapted to take up gaseous CO2 from the atmosphere. In the aquarium, placed under water, they are only able to use dissolved CO2. Several real water plants, meaning species, which are always under water in their natural habitat, are capable of also using the bicarbonate ion if CO2 availability is limited. It is not however favorable to allow this to happen, firstly because many other plants in the aquarium will be unable to grow. Secondly, with the use of bicarbonate ions the pH will rise to unacceptable levels for all plants.

Because of the CO2 equilibrium discussed, the maximum amount of dissolved CO2 present is dependent on the pH. The lower the pH the more CO2 present.

Since plants use CO2 in considerable amounts, they increase the pH at the same time. Thus the pH value and the CO2 concentration are interrelated.

Carbonate harness (KH), is yet another factor that effects the concentration of CO2. Hard water, with a high KH can hold more CO2 than soft water. While it is essentially true that hard water with a low pH holds the largest amount of CO2 in practice we can only use the values most suitable for plants, which are pH 6.4-7.2 and 3-8o KH.

The water plant enthusiast faces the task of maintaining both a stable pH and hardness in order to establish an optimum CO2 concentration. CLICK on our "Test Kits" link for our CO2 Test Kit.