Aquarium CO2 Guide: Do You Need It and How to Set It Up

Carbon dioxide is the single most transformative addition you can make to a planted aquarium. Plants use CO2 as their primary building block during photosynthesis — without adequate carbon, even perfect lighting and fertilisation cannot unlock a plant’s full growth potential. If you have ever wondered why competition aquascapes look so lush while your tank grows slowly and battles algae, the answer is almost certainly CO2.

But CO2 injection is not for everyone. It adds cost, complexity, and a degree of risk if mismanaged. This guide walks you through everything: whether you actually need CO2, how each system works, what equipment to buy, and how to set it all up safely in the context of Singapore’s tropical climate.

What CO2 Does for Aquarium Plants

Aquatic plants require three things to photosynthesise: light, nutrients, and carbon. In nature, carbon comes from dissolved CO2 in the water — produced by decomposing organic matter, fish respiration, and atmospheric exchange. In a closed aquarium, natural CO2 levels are typically very low, around 2–5 parts per million (ppm). Most aquatic plants grow best at 20–30 ppm.

When you bridge this gap with CO2 injection, the results are dramatic. Plants grow faster, produce more vibrant colours, develop compact and bushy forms, and — critically — outcompete algae for nutrients and light. A well-tuned CO2 system does not just make plants grow faster; it changes the entire balance of the ecosystem in favour of plants and against algae.

CO2 injection also lowers pH, which benefits species that prefer acidic conditions. For more on this relationship, see our guide on how to lower pH in your aquarium.

Do You Need CO2? Low Tech vs High Tech

Not every planted tank needs CO2 injection. The hobby broadly splits into two approaches:

Low Tech (No CO2)

Low-tech tanks use low to moderate lighting, rely on natural CO2 from fish respiration and atmospheric exchange, and grow undemanding plant species. Growth is slow, maintenance is minimal, and the look tends toward a mature, natural aesthetic. Suitable plants include Java fern, Anubias, Cryptocoryne, Vallisneria, Java moss, and Bucephalandra.

If you are happy with a simple, low-maintenance planted tank that still looks beautiful, you do not need CO2. Many stunning aquascapes have been created without it.

High Tech (With CO2)

High-tech tanks use intense lighting, pressurised CO2 injection, and comprehensive fertilisation to push plant growth to its maximum. This unlocks carpeting plants (HC Cuba, Monte Carlo, Glossostigma), intense red colouration (Rotala rotundifolia, Alternanthera reineckii), and the dense, manicured look of competition aquascapes. The trade-off is higher cost, more equipment, and a stricter maintenance schedule — weekly trimming and water changes are non-negotiable.

Choose high tech if you want to grow demanding species, achieve a specific aquascaping vision, or simply enjoy the hands-on aspect of optimising a complex system.

Pressurised CO2: The Gold Standard

A pressurised CO2 system uses a refillable cylinder of compressed carbon dioxide, regulated down to a controlled flow rate and dissolved into your aquarium water. It is the most reliable, consistent, and cost-effective method for long-term CO2 supplementation.

The basic components are:

1. CO2 cylinder — Aluminium or steel, typically 1 kg, 2 kg, or 5 kg capacity.
2. Regulator with needle valve — Reduces cylinder pressure (~800 psi) to a usable flow rate and allows fine adjustment.
3. Solenoid valve — An electromagnetic shut-off valve that connects to a timer, automatically turning CO2 on and off with your light schedule.
4. Bubble counter — A small chamber filled with water that lets you visually count the rate of CO2 bubbles per second.
5. CO2 tubing — Use CO2-resistant tubing (not standard airline tubing, which is permeable to CO2).
6. Diffuser or reactor — Dissolves CO2 into the water as a fine mist of microbubbles or via an in-line reactor.
7. Drop checker — A glass indicator filled with pH reagent that changes colour based on dissolved CO2 levels.

Equipment Breakdown

CO2 Cylinders

A 2 kg cylinder is the sweet spot for most home aquariums — small enough to fit inside a cabinet but large enough to last three to six months on a 60 cm tank. The 5 kg size is more economical per refill for larger setups. In Singapore, CO2 refills are available at welding gas suppliers and some aquarium shops for S$15–30 depending on size.

Regulators

Do not cheap out on the regulator — it is the most critical safety component. A quality dual-stage or single-stage regulator with a good needle valve prevents “end-of-tank dump,” a dangerous phenomenon where a nearly empty cylinder releases its remaining CO2 in a sudden burst. Reputable brands include UP Aqua, GLA, CO2Art, and Ista. Budget regulators work but tend to have less precise needle valves and cheaper solenoids.

Diffusers vs In-Line Reactors

Ceramic disc diffusers produce a fine mist of CO2 microbubbles inside the tank. They are simple and effective but require cleaning every few weeks as algae and mineral deposits clog the ceramic disc. In-line reactors connect to your canister filter’s return hose and dissolve CO2 completely before it enters the tank — no visible bubbles, no diffuser to clean, and more efficient dissolution. In-line reactors are the preferred option for serious planted tank enthusiasts.

Drop Checkers

Fill your drop checker with 4 dKH reference solution (not tank water) and add a few drops of pH reagent. The colour indicates CO2 concentration: blue means less than 20 ppm (too low), green means approximately 30 ppm (ideal), and yellow means above 40 ppm (potentially dangerous for fish). Place the drop checker on the opposite end of the tank from the diffuser for the most representative reading.

Setting Up Your CO2 System

1. Mount the cylinder upright in your aquarium cabinet or beside the tank. Secure it so it cannot topple.
2. Attach the regulator to the cylinder valve using the appropriate washer. Hand-tighten, then use a spanner for a quarter-turn more. Check for leaks by applying soapy water to connections — bubbles indicate a leak.
3. Connect the solenoid to a timer. Set CO2 to turn on one hour before your lights come on and off one hour before lights go off. This ensures CO2 levels are adequate when photosynthesis begins.
4. Run CO2-proof tubing from the regulator to the bubble counter, then to the diffuser or in-line reactor inside/on the tank.
5. Open the cylinder valve fully, then adjust the needle valve slowly. Start at one bubble per second for a 60-litre tank and increase gradually over several days.
6. Place the drop checker on the far side of the tank and observe the colour over 24–48 hours. Adjust the bubble rate until the drop checker reads green during the light period.
7. Monitor your fish. If they gasp at the surface or behave erratically, reduce the CO2 rate immediately.

DIY CO2 Systems

For hobbyists on a tight budget or with very small tanks, DIY CO2 offers a low-cost entry point. The most common method uses citric acid and baking soda in a dual-bottle reactor system. When citric acid is released into the baking soda bottle, it generates CO2 gas that is fed into the tank via tubing and a diffuser.

DIY CO2 kits are available at most local fish shops in Singapore for S$20–40. However, they come with significant limitations:

• Inconsistent CO2 output — pressure drops as the reactants deplete, making it difficult to maintain stable levels.
• No solenoid option — CO2 runs 24/7, which means it continues at night when plants are not photosynthesising, potentially lowering pH excessively and stressing fish.
• Frequent refills — the citric acid and baking soda mixture typically lasts only two to four weeks.
• Limited to small tanks — DIY systems cannot produce enough CO2 for tanks larger than about 80 litres.

DIY CO2 is a reasonable way to test whether you enjoy high-tech planted keeping before investing in a pressurised system, but it should not be considered a long-term solution.

Liquid Carbon (Seachem Excel)

Seachem Flourish Excel and similar liquid carbon products (glutaraldehyde-based) provide a bioavailable carbon source that plants can use in place of dissolved CO2. They are convenient — just dose daily — and also have mild algaecidal properties.

However, liquid carbon is not a true replacement for CO2 injection. It provides roughly the equivalent of 5–10 ppm CO2, far less than the 20–30 ppm a pressurised system delivers. It will help low-tech plants grow somewhat better and suppress algae, but it will not unlock the growth of demanding species or enable thick carpets.

Some plants are sensitive to glutaraldehyde — Vallisneria, certain mosses, and Elodea can melt or die with regular dosing. Start at half the recommended dose and observe for a week before increasing.

How Much CO2 to Inject

The target for most planted aquariums is 20–30 ppm of dissolved CO2. This is enough to drive excellent plant growth without endangering fish. Use the drop checker as your primary indicator: aim for a consistent green reading during the light period.

As a rough starting guide for bubble rate:

These are starting points only — actual requirements depend on tank dimensions, surface agitation (which off-gasses CO2), plant mass, and diffuser efficiency. Adjust based on drop checker readings, not bubble count alone.

Safety Considerations

CO2 overdose is a real risk. At concentrations above 40–50 ppm, fish begin to show distress — gasping at the surface, lethargy, erratic swimming. Prolonged exposure above 60 ppm can be fatal. Always:

• Use a solenoid on a timer so CO2 shuts off at night when plants are not consuming it.
• Never leave CO2 running when you are away for extended periods without monitoring.
• Maintain adequate surface agitation — a slight ripple helps off-gas excess CO2 and maintain oxygen levels.
• Place an air stone on a separate timer that runs at night to replenish oxygen while CO2 is off.
• Invest in a quality regulator to prevent end-of-tank dump.

End-of-tank dump occurs when a cylinder is nearly empty. The pressure drops below the regulator’s working range, and the remaining CO2 may be released in an uncontrolled burst. Dual-stage regulators virtually eliminate this risk. If you use a single-stage regulator, monitor the cylinder pressure gauge closely and refill when it drops below 400 psi.

Cost of CO2 in Singapore

Here is a realistic breakdown for setting up a pressurised CO2 system in Singapore:

The ongoing cost is remarkably low — a 2 kg cylinder refill every three to six months at S$15–25. Over a year, you spend less on CO2 gas than on fish food. The initial equipment investment pays for itself through healthier plants, less algae, and reduced need for algaecides and replacement plants.

CO2 systems, regulators, and accessories are available in our online shop.

Frequently Asked Questions

Can I run CO2 24/7 without a solenoid?

You can, but it is not recommended. At night, plants stop photosynthesising and produce CO2 through respiration instead of consuming it. Running injection overnight leads to excessively high CO2 by morning, which can drop pH dangerously and suffocate fish. A solenoid on a timer costs S$30–50 and eliminates this risk entirely.

Does CO2 injection harm fish?

At the correct level (20–30 ppm), CO2 is perfectly safe for all common aquarium fish. Problems arise only with overdosing or equipment malfunction. Fish that show signs of gasping or surface breathing should prompt an immediate check of your CO2 system. Use an airstone at night to ensure adequate oxygenation.

How do I know when my CO2 cylinder is running low?

Watch the pressure gauge on your regulator. A full 2 kg cylinder reads approximately 800–900 psi. The pressure remains fairly constant until the liquid CO2 inside is nearly gone, at which point it drops rapidly. Refill when the gauge reads 300–400 psi to avoid end-of-tank dump.

Is CO2 worth it for a small nano tank (under 20 litres)?

Yes, but the equipment can seem disproportionately large and expensive relative to the tank. A compact regulator with a small 500 g cylinder works well for nano setups. Alternatively, a DIY citric acid system or consistent liquid carbon dosing can provide adequate carbon for a small volume at lower cost.

A well-tuned CO2 system unlocks the full potential of your planted aquarium — transforming it from a simple fish tank into a thriving underwater garden. If you would like professional help designing and installing a high-tech planted setup, reach out to the Gensou team. We build custom aquariums with integrated CO2 systems, tailored lighting, and complete filtration — everything optimised from the start.

Related Reading

• CO2 in Your Aquarium at Night: What Happens and What to Do
• CO2, pH and KH: Understanding the Relationship
• Aquarium CO2 and pH Relationship: Reading the KH-pH Chart
• DIY CO2 for Aquariums: Citric Acid, Yeast and Budget Methods
• How to Balance Light and CO2 in a Planted Tank