Most aquarium plants grow fine without added CO2, but some plants grow faster and fuller with it

You do not have to inject CO2 into your aquarium for plants to survive. Many common aquarium plants—like Java fern, Anubias, Amazon sword, and Ludwigia—will grow in a standard tank with just the CO2 that naturally dissolves from the air and fish waste. However, plants that demand high light and nutrients (like Glossostigma, Dwarf hairgrass, and Rotala) will grow noticeably slower and thinner without supplemental CO2.

The real question is not whether you need CO2, but what you are trying to achieve. If you want a low-maintenance planted tank where plants grow steadily over months, skip the CO2 system. If you want dense, vibrant growth and are willing to maintain equipment, CO2 injection makes a visible difference within weeks.

Key Takeaways

  • Low-light plants like Java fern and Anubias grow without added CO2; high-light plants like Glossostigma and Rotala grow much slower without it.
  • Fish waste produces some CO2 naturally, and atmospheric CO2 dissolves into the water, so your tank is never at zero CO2.
  • A CO2 system requires a pressurized cylinder, regulator, diffuser, and ongoing refills, adding cost and maintenance to your tank.
  • Liquid carbon supplements (like Flourish Excel) are a cheaper alternative to pressurized CO2 but less effective for demanding plants.
  • High light, regular fertilizer, and water changes often matter more than CO2 for beginners trying to grow plants successfully.

Which plants actually need CO2 and which don't

Plants fall into two rough groups: low-demand and high-demand. Low-demand plants include Java fern, Anubias, Cryptocoryne, Amazon sword, and Ludwigia. These tolerate low light and low nutrients. They will grow in a tank with just fish waste and whatever CO2 is already present. Growth is slow—a few new leaves per month—but steady and reliable.

High-demand plants include Glossostigma, Dwarf hairgrass, Rotala, Ludwigia repens (red form), and Hemianthus callitrichoides (HC). These need bright light, stable nutrients, and CO2 to thrive. Without CO2, they grow spindly, pale, and slow. With CO2, they fill in dense and colorful. If you are new to planted tanks, avoid these plants unless you are ready to add a CO2 system.

Many plants sit in the middle—they grow without CO2 but noticeably better with it. Stem plants like Rotala rotundifolia and Ludwigia arcuata fall here. You can keep them without CO2, but they will not look as full or vibrant as they do in high-CO2 tanks.

How much CO2 is already in your tank

Your aquarium is not CO2-free even without injection. Fish respiration produces CO2, which dissolves into the water. Plant decay and uneaten food also release CO2. In a tank with fish and no plants, CO2 levels are usually around 3 to 5 parts per million (ppm). Low-demand plants can use this naturally occurring CO2 to grow, though slowly.

Water surface agitation—from filters, air stones, or powerheads—actually reduces CO2 by releasing it into the air. If you want to maximize natural CO2, reduce surface disturbance. If you want to add CO2, you will need to reduce agitation so the gas stays dissolved.

The more fish you have, the more CO2 your tank naturally produces. A heavily stocked tank with low-demand plants may grow them adequately without any supplemental system. A lightly stocked tank with the same plants will grow them more slowly.

What a pressurized CO2 system costs and requires

A basic pressurized CO2 setup includes four parts: a CO2 cylinder (usually 5 or 10 pounds), a regulator that controls gas flow, a diffuser that breaks the gas into tiny bubbles, and tubing to connect them. The cylinder sits outside or beside the tank. You also need a drop checker—a small glass device that tells you if CO2 levels are in the safe range (usually 20 to 30 ppm).

Initial cost ranges widely depending on quality, but a functional setup starts around $100 to $150 for a used cylinder and basic regulator, or $200 to $400 for new equipment. You then pay to refill the cylinder every few months to a year, depending on tank size and bubble rate. Refills typically cost $15 to $25 at aquarium shops or welding suppliers.

The system requires weekly monitoring: checking the drop checker, adjusting the regulator if needed, and watching for leaks. If the regulator fails or the cylinder empties, CO2 levels drop and plants suffer within days. This is manageable but not set-and-forget.

Liquid carbon as a cheaper alternative

Liquid carbon products like Flourish Excel and Thrive are much cheaper than pressurized CO2—usually $10 to $20 per bottle—and require no equipment. You dose them daily or weekly depending on the brand. They work by providing carbon in a different form that plants can use.

The trade-off is effectiveness. Liquid carbon helps low-demand plants grow a bit faster and can support some mid-tier plants, but it does not match pressurized CO2 for high-demand plants. Many experienced aquarists use liquid carbon as a supplement to pressurized CO2, not a replacement.

Liquid carbon also has a downside: it can harm some fish and invertebrates at high doses, particularly shrimp and certain fish species. Read the label carefully and dose conservatively if you keep sensitive animals.

Light and fertilizer often matter more than CO2

Before you buy a CO2 system, make sure your lighting and fertilizer are solid. Many beginners assume their plants are failing because of CO2 when the real problem is weak light or missing nutrients.

Light is the engine of plant growth. Low-demand plants need 8 to 10 hours of moderate light per day. High-demand plants need 10 to 12 hours of bright light. If your tank is lit by a single incandescent bulb or sits in a dim corner, no amount of CO2 will fix it. Upgrading to a proper LED light designed for plants often makes a bigger difference than adding CO2.

Fertilizer supplies the nitrogen, phosphorus, potassium, and trace minerals plants need. Fish waste provides some nutrients, but not always enough. All-in-one fertilizers like Thrive, Flourish All-in-One, or Estimative Index (EI) dosing can transform slow growth into visible progress within weeks. Start with fertilizer before CO2.

A tank with good light, regular fertilizer, and low-demand plants will outgrow a poorly lit tank with CO2 and high-demand plants. Get the basics right first.

When to add CO2 and when to skip it

Add pressurized CO2 if you want to grow high-demand plants, you have the budget for equipment and refills, and you are comfortable with weekly monitoring. This is the right choice for aquascapers building a specific planted layout or anyone who wants the fastest, most vibrant growth.

Skip CO2 if you are new to planted tanks, you want low maintenance, or you are growing low-demand plants. Start with good light and fertilizer. If your plants are thriving but you want them denser or faster, add liquid carbon first. Move to pressurized CO2 only if you decide you need it.

You can also run a tank without CO2 for years, then add it later if you want to upgrade. There is no penalty for starting straightforward and adding complexity later.

Frequently Asked Questions

Can I use an air pump instead of a CO2 system?

No. An air pump adds oxygen and removes CO2 from the water, which is the opposite of what you want. If you are adding CO2, you need to reduce or remove air stones and powerheads that agitate the surface.

What happens if I add too much CO2?

High CO2 levels (above 40 ppm) can stress or kill fish, especially sensitive species like tetras and shrimp. This is why a drop checker is essential—it keeps you in the safe zone of 20 to 30 ppm. If CO2 is too high, reduce the bubble rate or increase surface agitation.

Do I need CO2 if I have a lot of fish?

Not necessarily. A heavily stocked tank produces plenty of natural CO2 from fish waste. If you are growing low-demand plants, this may be enough. High-demand plants still benefit from supplemental CO2 even in heavily stocked tanks.

Can plants use the CO2 from my filter?

Filters do not produce CO2; they move water and remove waste. However, a filter that does not agitate the surface much will help retain the CO2 already in your tank, which is useful if you are trying to maximize natural CO2 without injection.

Is liquid carbon as good as pressurized CO2?

Liquid carbon is cheaper and easier but less effective. It works well for low-demand plants and can support some mid-tier plants. For high-demand plants that need fast, dense growth, pressurized CO2 is noticeably better.