Should You Turn Off CO2 at Night? The Science Explained

Table of Contents

• The Short Answer
• The Science: Photosynthesis and Respiration
• Why You Should Turn Off CO2 at Night
• Setting Up a Solenoid Timer
• The Optimal CO2 Schedule
• Surface Agitation at Night
• Understanding Drop Checker Lag
• Are There Any Exceptions?
• Frequently Asked Questions

The Short Answer

Yes, you should turn off CO2 injection at night. Plants do not use CO2 in the dark, so injecting it overnight wastes gas, can suffocate fish, causes unnecessary pH swings, and serves no benefit to your planted aquarium. A solenoid valve on a timer automates this entirely.

Now let us explore the science behind this recommendation and how to set up your system correctly.

The Science: Photosynthesis and Respiration

To understand why CO2 should be off at night, you need to understand two fundamental biological processes that your aquarium plants perform:

Photosynthesis (During Light Hours)

When your aquarium light is on, plants absorb CO2 and water, using light energy to produce glucose (sugar) and oxygen. This is why we inject CO2 — to ensure plants have enough of this critical raw material to grow vigorously and outcompete algae.

The simplified equation:

CO2 + Water + Light Energy → Glucose + Oxygen

During the day, your plants are net oxygen producers. They release oxygen into the water, benefiting your fish and other inhabitants.

Respiration (24 Hours, But Dominant at Night)

All living organisms, including plants, respire continuously. Respiration is essentially the reverse of photosynthesis — organisms consume oxygen and release CO2 to generate energy from stored glucose.

Glucose + Oxygen → CO2 + Water + Energy

During the day, photosynthesis vastly outweighs respiration, so plants are net consumers of CO2 and net producers of oxygen. At night, with no light, photosynthesis stops entirely. Plants continue to respire, consuming oxygen and releasing CO2 — just like your fish do all the time.

Why You Should Turn Off CO2 at Night

Understanding the science makes the reasons clear:

1. Plants Cannot Use CO2 Without Light

CO2 is only consumed during photosynthesis, which requires light. Injecting CO2 into a dark tank is like pouring petrol into a parked car with the engine off — it simply accumulates with nowhere to go. The CO2 dissolves into the water, building up to levels far beyond what plants can use when the light eventually comes back on.

2. Fish Can Suffocate

This is the most critical safety concern. At night, your fish, invertebrates, bacteria, and plants all consume oxygen and produce CO2. If you are also injecting CO2 from your pressurised system, the dissolved CO2 level rises dramatically while dissolved oxygen plummets.

In Singapore’s warm water (28–32°C), this problem is compounded. Warm water holds less dissolved oxygen than cool water. A heavily planted tank with CO2 running overnight can reach dangerously low oxygen levels, and you may find fish dead or gasping at the surface in the morning.

3. Wasteful and Costly

CO2 refills are not free. In Singapore, refilling a standard 2 kg CO2 cylinder costs approximately $15–$25, and a 5 kg cylinder runs $25–$40. Running CO2 around the clock uses roughly 40–50% more gas than running it only during the photoperiod. Over a year, this adds up to several extra refills — money spent for zero benefit.

4. pH Instability

Dissolved CO2 forms carbonic acid in water, lowering pH. When CO2 runs 24/7, pH drops at night (when plants are not consuming it) and rises during the day (when plants absorb it). This creates a larger daily pH swing than necessary. Fish prefer stable pH — the magnitude of the swing matters more than the actual number. Turning CO2 off at night produces a more gradual, smaller pH fluctuation.

Setting Up a Solenoid Timer

A solenoid valve is an electromagnetic valve that sits on your CO2 regulator. When energised (power on), it opens and allows CO2 to flow. When de-energised (power off), it closes and stops CO2 flow. Connecting it to a timer automates the on/off cycle.

What You Need

Most quality CO2 regulators sold in Singapore (UP Aqua, ISTA, Intense, CO2Art) come with a built-in solenoid. If yours does not, standalone solenoid valves can be added inline.

Installation Steps

1. Ensure your CO2 regulator has a solenoid valve (the part with an electrical plug).
2. Plug the solenoid into a timer (mechanical pin timers from hardware shops work perfectly).
3. Set the timer to turn on one hour before your aquarium light comes on.
4. Set the timer to turn off one hour before your light goes off.
5. Verify that the solenoid clicks shut when the timer turns off and that no bubbles appear in your bubble counter.

For more detail on setting up timers for all your aquarium equipment, see our aquarium timer guide.

The Optimal CO2 Schedule

The ideal CO2 schedule accounts for the time it takes for CO2 to build up in the water column and the lag in the drop checker:

Why Turn CO2 on Early?

CO2 takes time to dissolve and distribute throughout the water column. If you turn CO2 on at the same time as your light, your plants spend the first hour or two of the photoperiod in a CO2-deficient state. This inefficiency can promote algae, which thrives when CO2 is low relative to light intensity. Starting CO2 one hour early ensures your plants hit the ground running.

Why Turn CO2 off Early?

Residual dissolved CO2 remains in the water for some time after injection stops. Turning CO2 off one hour before lights off means your plants still have adequate CO2 for that final hour, but levels begin declining naturally. By the time the lights go off, CO2 is already dropping, and oxygen recovery begins immediately — exactly what your fish need for the night.

Surface Agitation at Night

With CO2 off at night, you can and should increase surface agitation to boost oxygen exchange. Some hobbyists use a simple approach:

• Air pump on a timer: Set an air pump to turn on when the CO2 turns off and off when the CO2 turns on. This provides maximum oxygenation at night without degassing CO2 during the photoperiod.
• Adjustable filter outlet: Lily pipes or directional outlets can be angled to break the surface slightly, providing consistent gas exchange without an additional air pump.
• Surface skimmer: A surface skimmer running 24/7 provides gentle surface agitation that aids oxygenation without excessively degassing CO2 during the day.

In Singapore’s warm climate, where dissolved oxygen levels are already lower due to higher water temperatures, nighttime aeration is particularly important for heavily stocked or densely planted tanks.

Understanding Drop Checker Lag

A drop checker (the small glass device filled with indicator solution that shows green, blue, or yellow) is the standard tool for monitoring CO2 levels in planted tanks. However, it has an important limitation: it lags behind actual CO2 levels by approximately one to two hours.

This means:

• When you turn CO2 on in the morning, the drop checker will still show blue (low CO2) even as CO2 levels are rising. It takes one to two hours to reflect the actual level.
• When you turn CO2 off in the evening, the drop checker will remain green (good CO2) for an hour or more, even as actual CO2 levels are declining.
• The drop checker never shows you real-time CO2 levels. It shows you what CO2 levels were an hour or two ago.

This lag is why the one-hour-early CO2 start is so important. By the time your lights come on and plants need CO2, the actual dissolved level has already reached the target — even if the drop checker has not caught up yet.

Do not adjust your CO2 injection rate based on the drop checker’s colour immediately after turning the system on or off. Make adjustments based on the mid-photoperiod reading (three to four hours after lights on), when the drop checker most accurately reflects the actual CO2 concentration.

Are There Any Exceptions?

There are very few legitimate reasons to run CO2 at night:

• pH controller systems: Some advanced setups use a pH controller that automatically stops CO2 when pH drops below a set point. In theory, these could run 24/7 safely because the controller prevents overdosing. In practice, even with a pH controller, running CO2 at night is wasteful since plants are not using it.
• Very lightly stocked tanks with few plants: If your tank has minimal bioload and sparse planting, overnight CO2 accumulation may not reach dangerous levels. However, it is still wasteful, and a timer costs less than the extra CO2 refills.

In virtually every scenario, turning CO2 off at night is the correct choice. The cost of a solenoid and timer is recovered within a few months in saved CO2 refills alone, and the safety benefits for your fish are invaluable.

Frequently Asked Questions

What happens if I forget to turn CO2 off at night?

A single night is unlikely to cause a disaster in a lightly stocked tank with good surface agitation. However, in a heavily stocked tank or one with minimal water movement, overnight CO2 injection can drop oxygen levels low enough to stress or kill fish. The risk increases with each consecutive night of CO2 running. This is precisely why a solenoid timer is so important — it removes the human error factor entirely. A one-time investment of $20–$30 for a timer is cheap insurance against fish loss.

Can I just reduce the CO2 rate at night instead of turning it off?

You could, but there is no benefit. Plants use zero CO2 in the dark, so even a reduced injection rate is entirely wasted. The only approach that makes sense is a complete stop. Additionally, adjusting the needle valve twice daily is impractical and introduces inconsistency. A solenoid provides a clean on/off that requires no daily adjustment.

Will my plants suffer from not having CO2 at night?

Absolutely not. Plants do not consume CO2 at night because photosynthesis requires light. At night, plants are in respiration mode only — consuming oxygen and releasing CO2, just like animals. Your plants will not experience any negative effects from having CO2 off during dark hours. This is how it works in nature as well; CO2 levels in natural waterways fluctuate between day and night.

Do I need a solenoid, or can I just manually turn the CO2 cylinder on and off?

You can manually open and close the main valve on your CO2 cylinder, but this is impractical and risks inconsistency. Each time you close and reopen the valve, the needle valve setting may shift slightly, changing your bubble count. Over time, this leads to fluctuating CO2 levels, which promote algae. A solenoid operates downstream of the needle valve, so your bubble rate remains perfectly consistent every time it opens. The modest cost of a solenoid and timer is one of the best investments in a planted tank setup.

Expert Planted Tank Setup

Getting your CO2 system right is one of the most important steps in planted tank success. At Gensou Aquascaping, we have been designing and maintaining high-tech planted aquariums in Singapore for over 20 years. From CO2 system installation to complete aquascape design and build, we ensure every detail is optimised for healthy plant growth and fish safety.

Visit us at 5 Everton Park, Singapore, or contact us to discuss your planted tank project. Let us help you grow the aquascape you have always envisioned.