CO2 additive: what you need to know

What is a CO2 additive?

CO2 additives are liquid products (e.g. Seachem Excel, Easy Carbon, API CO2 Booster) sold as alternatives to injected CO2. They are not dissolved CO2 gas. The main active ingredient is usually glutaraldehyde — a biocide that can be metabolised by some plants into a carbon source. The name “liquid CO2” is marketing; it’s misleading but widely used.

What do they actually do?

• Carbon source: Some plants can use glutaraldehyde-derived carbon. In low-tech tanks, this may give a modest growth boost.
• Algae control: Glutaraldehyde is an algaecide. It can suppress certain algae (e.g. hair algae, BBA) when used as directed. Overdosing harms plants and fish.
• Not equivalent to injected CO2: They do not supply the same amount or form of carbon as a proper CO2 system. High-tech planted tanks rely on real CO2 injection.

When they help

In low-tech or low-light tanks, a CO2 additive may slightly improve plant growth and help keep algae in check. They’re easier than setting up a CO2 cylinder and diffuser. For undemanding plants (java fern, anubias, moss), the benefit is often small — good light and fertiliser matter more.

When they fall short

For fast-growing, demanding plants (e.g. carpeting plants, red plants, many stems), injected CO2 is far more effective. Additives cannot match the carbon availability of a proper CO2 system. If you want a high-tech aquascape, plan for real CO2.

Safety and dosing

• Dose according to the label. Overdosing can damage plants, harm shrimp and invertebrates, and stress fish.
• Some species (e.g. vals, elodea) are sensitive to glutaraldehyde and may melt.
• Shrimp and snails can be more sensitive than fish. Start with a lower dose if you keep inverts.

Quick takeaways

• CO2 additives are glutaraldehyde-based, not dissolved CO2. The name is misleading.
• They can modestly boost growth and help with algae in low-tech tanks.
• They do not replace injected CO2 for demanding plants.
• Dose carefully; overdosing harms plants and inverts.