CO2 in Your Aquarium at Night: What Happens and What to Do
This aquarium CO2 at night guide answers a question every planted tank hobbyist in Singapore eventually asks: what happens to carbon dioxide levels when the lights go off, and should you do anything about it? At Gensou Aquascaping, 5 Everton Park, we walk customers through the science and the practical steps to keep their tanks safe and thriving around the clock.
How CO2 Behaves During the Day
During the photoperiod, aquatic plants photosynthesise, absorbing carbon dioxide from the water and releasing oxygen. In a well-planted, CO2-injected tank, the injection rate is calibrated so that plants receive a steady supply of carbon while maintaining safe levels for fish and invertebrates. Oxygen levels rise throughout the day as plants actively produce it. This daytime balance is the foundation of a healthy planted aquarium, and most hobbyists become comfortable managing it relatively quickly.
What Changes at Night
When the lights turn off, photosynthesis stops, but respiration continues. Plants, fish, invertebrates and bacteria all consume oxygen and produce carbon dioxide around the clock, including during the dark period. Without photosynthesis to absorb CO2, levels in the water rise steadily through the night. Simultaneously, oxygen levels drop because nothing is producing it while every living organism in the tank continues to consume it. This natural shift is manageable in most tanks, but it can become dangerous if CO2 injection continues to run after lights-out.
Risks of Running CO2 at Night
If your pressurised CO2 system remains active after the photoperiod ends, carbon dioxide accumulates in the water without being consumed by plants. This drives pH downward and can reach concentrations that stress or kill fish and shrimp. Symptoms of CO2 overdose include fish gasping at the surface, erratic swimming, lethargy and, in severe cases, death. Shrimp are particularly sensitive and may show distress before fish do. The risk is highest in smaller tanks where gas concentrations change rapidly and in heavily stocked systems where oxygen demand is already high. Never assume that a level safe during the day remains safe at night when the dynamics shift.
Should You Turn CO2 Off at Night
Yes. The standard and recommended practice is to run your CO2 injection on a timer that switches off at least 30 minutes to one hour before your lights turn off. This allows residual CO2 in the water to begin dissipating before photosynthesis ceases entirely. Some hobbyists also set CO2 to turn on one to two hours before lights-on, so that the target concentration is reached by the time photosynthesis begins. A simple plug-in timer or smart plug, readily available in Singapore, handles this automation reliably. The cost of a timer is negligible compared to the risk of losing livestock to overnight CO2 accumulation.
The Role of Surface Agitation and Aeration
Surface agitation drives gas exchange between the water and the atmosphere. During the day, excessive agitation is counterproductive because it off-gasses the CO2 you are injecting. At night, however, increased surface agitation helps dissipate accumulated CO2 and replenishes dissolved oxygen. Many hobbyists run an air pump on a separate timer that activates when the CO2 and lights turn off. This provides supplementary aeration through the dark period. An airstone positioned to create gentle surface rippling is sufficient. In Singapore’s warm climate, where dissolved oxygen capacity is lower due to higher water temperatures, nighttime aeration is especially valuable.
pH Swings Between Day and Night
CO2 dissolved in water forms carbonic acid, which lowers pH. During the day, as plants consume CO2, pH rises. At night, as CO2 accumulates, pH falls. A swing of 0.5 to 1.0 pH units between day and night is normal in a CO2-injected planted tank and generally well tolerated by most fish species. However, swings larger than 1.0 unit indicate either excessive CO2 injection or insufficient buffering capacity (KH). If your KH is very low, even moderate CO2 levels can cause dramatic pH drops. Test pH at different times of day to understand your tank’s cycle and adjust injection rates or KH buffering accordingly.
Monitoring Tools and Best Practices
A drop checker filled with 4 dKH reference solution provides a visual indication of CO2 concentration. Check its colour at lights-on, mid-photoperiod and lights-off to understand the daily CO2 curve in your tank. Lime green indicates approximately 30 parts per million, which is the target for most planted tanks. If the drop checker is yellow at any point, CO2 is too high and poses a risk to livestock. A pH meter or continuous pH monitor offers more precise data for hobbyists who want detailed control. At Gensou Aquascaping, we help customers set up their CO2 timing and monitoring systems for peace of mind.
Practical Setup Summary for Singapore Hobbyists
A reliable nighttime routine for a planted tank in Singapore involves the following: set CO2 to turn on one to two hours before lights-on and off 30 to 60 minutes before lights-off. Run an air pump from lights-off until lights-on to maintain oxygen levels through the warm tropical night. Use a drop checker to verify CO2 levels remain within a safe range. Maintain a KH of at least 2 to 3 dKH to buffer against excessive pH swings. Avoid overstocking, which compounds overnight oxygen depletion. With these measures in place, your fish, shrimp and plants coexist safely through every 24-hour cycle. For CO2 equipment, timers and personalised setup advice, visit Gensou Aquascaping at 5 Everton Park.
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5 Everton Park #01-34B, Singapore 080005 · Open daily 11am – 8pm
