Algae are plants, but only very simple ones. Their demands on light and nutrients are very similar to those of our aquarium plants. And that is exactly why aquarium plants and algae are food competitors. When the plants grow well, there is hardly any food left for the algae and they wither away. CO2 fertilisation promotes the growth of the other plants and algae have no chance to grow! In aquariums with fewer plants, as is often the case in Malawi-Tanganyika Lake aquariums, the few plants there need to be maintained all the more intensively in order to actively combat algae growth. By the way, light plays a very subordinate role in algae control compared to the supply of nutrients! This is a very important insight, because people often blame algae growth on the quantity or quality of the light. To demonstrate this: Place a canister of distilled water in the blazing sun. Algae will never form in it, because the distilled water contains no nutrients. The light only represents the engine for algae growth. The fuel for the engine, however, are the nutrients nitrate and phosphate.
Does CO2 fertilisation mean less oxygen in the water?
A lot of people believe that CO2 reduces the oxygen content in the water. This is not correct! Nevertheless, let’s explain the connection in more detail. If, despite adding CO2, you move the water surface a lot with bubbling stones or spray bars from the filter, the oxygen content in the water is increased, but at the same time the CO2 is expelled again (it’s like shaking a cola bottle). The calmer the water surface, the more CO2 remains in the water. Both gases (CO2 and O2) can be present in the water in high concentrations, e.g. if the plants assimilate strongly during the day (then a lot of O2 is produced) and at the same time a lot of CO2 is added via a CO2 fertiliser system. At night it’s better to switch off the CO2 system (via a solenoid valve), as plants do not consume CO2 during the dark phase and adding CO2 at night would only unnecessarily lower the pH value, which already drops at night. In heavily planted aquariums it may be advisable to run an additional aeration system at night, as the oxygen consumption of the aquatic plants can lead to an oxygen deficit in the water. To find out you can test the oxygen content of the water in the morning before the light switches on. If the O2 content is still above 5 mg/l in the morning, there is no danger for fish and invertebrates.