What CO2 concentration is correct?

Dennerle recommends a CO2 concentration in the aquarium of between 15 and 30 mg/litre – a value between 20 and 25 mg/litre is ideal. The dosage required for this CO2 concentration depends on various factors such as planting, water flow, surface/volume ratio etc. This is why it is necessary to individually calculate the CO2 dosage for every aquarium.  

How do I determine the CO2 concentration in the water?

CO2 concentration, carbonate hardness (KH) and pH are in a certain ratio to one another. The CO2 concentration of the water can be calculated from the pH and carbonate hardness. Suitable pH and KH tests can be obtained from specialist retailers.
• Measure the carbonate hardness of your aquarium water.
• Read the corresponding pH for the desired CO2 concentration in the table. You should adjust the pH +/- 0.1 by appropriately regulating the bubble count. Example: Carbonate hardness 4 °d, recommended pH 6.8 +/-0.1.

How do I calculate the required dosage?

Rule of thumb for the basic adjustment of the bubble count: 10 bubbles per minute per 100 l aquarium water.
Example: 200 l aquarium: 2 x 10 = 20 bubbles per minute.
Adjust the added CO2 in small steps until the desired CO2 concentration is reached. Please note: The more the water surface is moving (e.g. due to the filter, additional aeration) the more CO2 is driven out of the aquarium.

CO2 concentration (mg/l) dependent on carbonate hardness (°d) and pH

Use the table as follows:

The figures highlighted in green are optimum for healthy, strong plant growth.
Measure the carbonate hardness, e.g. 5°dH and look for a medium CO2 value (green area), e.g. 20 mg/l. Then you can read the corresponding pH of 6.9 at the top and set it as a target value with the pH controller.