Carbonic acid equals carbon dioxide (CO2, popularly known as carbon dioxide) is very important for the growth of aquatic plants. If enough CO2 is available to aquatic plants in free form, they can grow well, converting organic compounds and releasing oxygen under the influence of light. This is the assimilation process. The CO2 content is not constant during a 24-hour period, by the way. It fluctuates under the influence of day and night (assimilation process). Underwater plants absorb CO2 and release oxygen with the help of light. The CO2 content therefore decreases as the day progresses. At night, the process is reversed; plants take in oxygen (albeit limited) and release CO2 (albeit limited). Microorganisms, the largest producers of CO2, produce both during the day and at night. This means that CO2 will accumulate at night and decrease significantly in the afternoon. By the way, this process occurs only in well-functioning water bodies, that is, when there is a growing stock of underwater plants and sufficient activity of the microorganisms. With the pH value, we can control this process. If the pH is relatively low early in the morning (pH 7-8) and relatively high in the evening (pH 8-9) then the pond environment is functioning well. The plants will grow well and the water will be clear. However, if the pH is equally high in the morning and in the evening (pH 9 or higher), there is stagnation: the submerged plants will not grow and the picture is a pond full of algae. After the growing season, the pH will drop. The growth of aquatic plants (and algae) stagnates. Thus, they no longer absorb CO2. The CO2 level in the pond will therefore slowly increase. If the GH value (see chapter GH) is high enough, the excess CO2 can bind to carbonate and the water will not acidify. The pH value will not drop below pH 7. However, if the GH value is too low, there is insufficient calcium to bind the excess CO2. The water becomes acidic and oxygen deficiency occurs. The pH value in this case drops to values below pH 6. With a simple Aqua test set, the pH value can be determined very precisely.