How Much CO2 Do Aquarium Plants Need? A Complete Guide

CO2 is the single most transformative thing you can add to a planted aquarium. Get it right and your plants will grow faster, display richer colours, and outcompete algae. Get it wrong — too little or too much — and you'll either see no benefit or, at the extreme, stress your fish. This guide cuts through the confusion.

Why CO2 matters so much

Plants build their entire structure — leaves, stems, roots — from carbon. In nature, dissolved CO2 in rivers and lakes provides this continuously. In a closed aquarium, CO2 is quickly depleted by plants during daylight hours and may never be replenished to adequate levels by surface gas exchange alone. The result is plants that are perpetually carbon-starved: slow-growing, prone to algae attack, and unable to make use of the light and nutrients you're providing.

Adding CO2 doesn't just grow plants faster — it unlocks the full potential of everything else in your tank. Fertilisers work better. Colours intensify. Carpeting plants actually carpet. The tank transforms.

How much CO2 do you need?

The target for a high-tech planted tank is typically 25–35 mg/L of dissolved CO2. In practice, you can't easily measure this directly, which is why a drop checker is the standard tool for monitoring CO2 in real time.

A drop checker contains a reagent solution that changes colour according to the pH of the water, which in turn reflects CO2 concentration:

• Blue — CO2 too low, plants are carbon-starved
• Green — good CO2 levels, around 25–30 mg/L
• Yellow-green — high CO2, still acceptable if fish and shrimp are behaving normally
• Yellow — CO2 very high, monitor fish carefully for distress

The goal for most planted tanks is a steady green. In practice, running CO2 a little higher — into yellow-green — can produce dramatically better plant growth, provided your fish and shrimp are showing no signs of distress (gasping at the surface, laboured breathing, unusual lethargy).

If you have no livestock, you can push CO2 higher still. Without the constraint of fish welfare, you'll be amazed at how aggressively plants grow when CO2 is in genuine abundance.

Pressurised CO2 vs liquid carbon

Pressurised CO2 injection

A pressurised system — CO2 cylinder, regulator, solenoid, diffuser — delivers dissolved CO2 directly and consistently into the water column. This is the most effective approach and the one you need for high-light planted tanks with demanding species like carpeting plants, Rotala, and Hemianthus. The solenoid connects to a timer so CO2 comes on with your lights and switches off overnight (when plants aren't photosynthesising and CO2 can build to harmful levels).

Liquid carbon

Liquid carbon products like Neutro CO₂ and EasyCarbo provide an alternative carbon source in a bottle — dosed daily like a fertiliser. They're not a direct substitute for pressurised CO2, but they do provide carbon that plants can use, and they have a useful algae-suppressing effect as a side benefit.

Liquid carbon is a good choice for low-to-medium tech setups with hardy, undemanding plants. It's considerably cheaper to get started with and doesn't require any equipment. For many hobbyists, it's the right tool — just don't expect it to unlock the same growth rates as pressurised CO2.

CO2 timing: when to run it

For pressurised systems, set your solenoid to switch CO2 on about one hour before your lights come on. This gives dissolved CO2 time to build up in the water before photosynthesis begins. Switch CO2 off when your lights go off — overnight, plants respire rather than photosynthesise and don't consume CO2, so it will accumulate to potentially harmful levels if left running.

A practical schedule: lights on at 12:00, CO2 on at 11:00. Lights off at 19:00, CO2 off at 19:00. Adjust to suit your tank, but the principle is the same: CO2 leads the light period by an hour and ends with it.

Signs your CO2 is too low

• Drop checker consistently blue
• Slow or stalled plant growth despite good light and fertiliser
• Persistent algae, especially green spot or thread algae
• Plants not pearling (producing visible oxygen bubbles)

Signs your CO2 is too high

• Fish gasping at the water surface, particularly in the morning
• Shrimp behaving erratically or clustering near the surface
• Drop checker consistently yellow

If you see distress signs, turn CO2 off immediately and increase surface agitation to off-gas CO2 quickly. Then dial back your regulator and find the right balance more gradually.

Frequently asked questions

How much CO2 per hour should I inject?

There's no single answer — the right bubble rate depends on your tank volume, diffuser efficiency, circulation, and how well your tank is sealed. Start low, monitor your drop checker, and increase gradually until you see a consistent green. A typical starting point is 1–2 bubbles per second for a 60-litre tank, but adjust based on what your drop checker tells you.

Is CO2 safe for fish?

At correct levels, yes — fish live alongside dissolved CO2 naturally. Problems only arise when CO2 gets too high, which displaces oxygen. Keep your drop checker in the green-to-yellow-green range and watch your fish; distress signs are obvious and CO2 disperses quickly if you increase surface movement.

Can I use CO2 with a drop checker only?

A drop checker is the standard and recommended tool for monitoring CO2 in a planted tank. Note that it measures historical CO2 levels with a slight lag (the reagent takes time to equilibrate), so use it as a guide and combine with observation of plant pearling and fish behaviour for a complete picture.

Do I need CO2 for a low-tech planted tank?

No. Many species — Anubias, Java Fern, Cryptocoryne, mosses — thrive without injected CO2. Use liquid carbon as an alternative and match your light to a lower-demand level. You'll have a beautiful, healthy planted tank without the complexity of pressurised CO2.