Diurnal pH Fluctuation

pH CO2 fluctuation day and night

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In a well-balanced aquarium, the interactions between living organisms and the aquatic environment form a dynamic ecosystem. During the day, as light penetrates the water, aquatic plants undergo photosynthesis, converting carbon dioxide and water into oxygen and glucose. This process releases oxygen into the water, which helps maintain a stable pH level by absorbing the carbon dioxide exhaled by fish and other aquarium inhabitants.

However, as the sun sets and photosynthesis subsides, the situation changes. Photosynthetic organisms like plants and corals continue to respire, utilizing the energy stored during the day. This respiration process consumes oxygen and releases carbon dioxide into the water. Simultaneously, the fish and other aquarium inhabitants also respire, contributing to the release of CO2. In a closed aquarium, this excess CO2 can actually have an impact on the aquarium’s pH, recall that carbon dioxide dissolved in water forms a weak acid called carbonic acid.

This happens in aquariums absent a large fish bio load as well. When plants ultimately use the glucose produced during photosynthesis, they consume oxygen and release carbon dioxide just as animals do when they respire. As a matter of fact, plants respire nonstop, both during the day and at night as well.

The combined effect of these respiratory processes and the absence of photosynthesis to reabsorb all that carbon dioxide can lead to an accumulation of CO2 in the aquarium, causing the pH to drop whenever the plants aren’t photosynthesizing.

During the period when the lights are off and photosynthesis has ceased, not only is there a noticeable increase in the carbon dioxide concentration, but the aquarium will have a noticeable and corresponding decrease in oxygen concentrations as well, which can also have a detrimental impact on aquarium inhabitants.

This process is primarily an issue in tanks heavily stocked with plants and a healthy selection of fish or other animals, and is primarily a problem in soft-water setups like aquascapes. This is because the low alkalinity of these systems exacerbates this process causing larger fluctuations than on systems with a higher degree of buffering. However, that doesn’t mean that tanks with elevated alkalinity don’t experience similar CO2 and pH fluctuations.

In the figure below (credit: Treefer32) you can see a real-time chart of both pH (orange) and light intensity (green) in a heavily stocked coral reef aquarium. Though the pH fluctuation is relatively small, and likely not a problem for this system, you can clearly see how predictable this fluctuation can be. Also notice that the peak pH occurs at the end of the photoperiod, after ramping up all day, and the corresponding minimum occurs a few hours before the lights typically turn back on. It is also pertinent to note that activities inside the house can impact the carbon dioxide levels in your tank. New air-tight construction prevents gas exchange with the outside air and burning methane as in a gas range can impact CO2 and pH significantly.

To mitigate diurnal pH fluctuations, potential carbon dioxide overdose, and a decrease in oxygen that can be potentially deadly to animals such as fish or shrimp, aquarists can adopt several strategies:

CO2 Injection Timing

The easiest way to prevent oversaturation of carbon dioxide in your aquarium is to avoid dosing it after the lights go out. Adding a timer and a solenoid valve to the CO2 regulator enables easy control, ensuring CO2 injection only occurs during daylight hours. This straightforward approach helps maintain a stable and safe environment for your aquatic inhabitants.

Implementing Aeration

Ensuring adequate aeration during the night is crucial for countering CO2 oversaturation and help stabilize pH in your aquarium. Employing air stones to surface agitation helps maintain good gas exchange to provide adequate oxygen levels and reduces the impact of CO2 accumulation. Operating these devices on a timer, synchronized with your lighting feature, allows them to activate after the lights go out and switch off when the lights come back on the following day. This helps maintain a balanced and oxygen-rich aquatic environment, providing a healthy habitat for your fish and plants. By incorporating proper aeration, you can effectively mitigate diurnal pH fluctuations.

Regulate CO2 with a pH Controller

Precision regulation of carbon dioxide delivery can be a game-changer for planted aquarium enthusiasts. Incorporating a pH controller offers the aquarist a proactive solution to prevent diurnal fluctuations in CO2 concentration by precisely dosing CO2 to a predetermined pH setpoint.

A pH controller can continuously monitor the pH level in the water. When the pH starts to drop below the pre-set setpoint, indicating an increase in CO2 concentration in the aquarium, the controller de-activates the CO2 injection system to prevent an overdose of carbon dioxide, ensuring that the aquarium’s pH remains stable and balanced. This real-time feedback mechanism enables precise dosing of CO2 to match the aquarium’s demand, eliminating excess CO2 buildup and potential stress on aquatic life.

Maintaining a successful aquarium means understanding the delicate balance between photosynthesis, respiration, and the CO2 concentration in your aquarium. As long as you remain cognizant of the potential impacts of CO2 oversaturation, preventing significant issues becomes a manageable task.


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