CO2 (carbon dioxide)

CO2 is the number one nutrient for aquatic plants. Without an adequate CO2 source, plants grow poorly and algae take over.

Detailed explanation

In planted fresh water, CO2 is fundamental. Plants use it for photosynthesis. In nature, CO2 comes from organic decomposition in the substrate, from fish respiration, from exchanges with the atmosphere. In the aquarium these sources are rarely enough for demanding plants.

A pressurized CO2 system with cylinder, reducer, solenoid valve and diffuser is the standard for Dutch aquariums and aquascaping. CO2 should only be supplied during the photoperiod (when the light is on). At night, plants do not photosynthesize and CO2 would accumulate, lowering the pH.

The ideal CO2 level is approximately 20-30 ppm. It occurs with a drop checker (reagent indicator) that changes colour: green = optimal level, blue = too little, yellow = too much.

Warning: too much CO2 kills fish. Shrimps are the most sensitive. You always need a solenoid valve connected to the light timer and a good drop checker for control.

In marine aquariums, CO2 is never added. Indeed, we try to eliminate it because it lowers the pH. The skimmer and refugium help exchange CO2 with the atmosphere.

CO2 is not used in ponds. Marsh plants have emerged leaves and take CO2 from the air.

Reference values by aquarium type

How it is measured

Drop checker with 4 dKH solution and pH indicator. Green = 20-30 ppm. Alternatively, it is calculated from the pH/KH table by crossing the measured values.

What happens when it is too low

Plants that grow slowly. Small leaves. Algae that take over because they compete better than plants at low CO2 concentrations.

What happens when it is too high

Surface-breathing fish. Shrimps moving frantically. pH that drops below 6. In serious cases mortality.

Related terms