CO2 for planted tanks: pressurised diffuser or liquid carbon?
A planted tank with no CO2 supplementation quickly hits a ceiling: light alone cannot grow demanding plants if carbon is in short supply. A study by James Titus, published in 2017 in the journal Ecology, shows that dissolved CO2 availability is often the main factor limiting the growth of submersed aquatic plants, more so than light in many natural settings (study on Google Scholar). That helps frame the choice between a pressurised CO2 kit and liquid carbon. Find all our equipment picks in the accessories section.
Why aquatic plants run short of CO2
CO2 diffuses roughly ten thousand times more slowly in water than in air: a submerged plant naturally has access to far less carbon than a land plant, even under strong light. Powerful lighting with no CO2 supply throws the tank out of balance: plants cannot make use of all that light, and algae take advantage.
The pressurised CO2 kit
A CO2 cylinder (30 to 80 € for a first setup including regulator and diffuser, then refills at roughly 15 to 25 €), an adjustable-flow regulator and a diffuser or reactor allow continuous CO2 injection, usually timed to match the lighting schedule. It is the most effective option for a densely planted tank, but it calls for careful tuning: too much CO2 overnight, once plants stop consuming it, can suffocate fish.
Liquid carbon, a simpler alternative
Based on glutaraldehyde, liquid carbon (8 to 20 € a bottle) is dosed daily with a pipette. Less effective than pressurised injection, it suits lightly to moderately planted tanks or beginners who would rather not invest in a cylinder. One caution: at high doses it is toxic to shrimp and some aquatic mosses, so always follow the labelled dose and start at half strength for the first week.
Which to choose for your tank
- Lightly planted tanks, or anubias/ferns: no CO2 supplementation is needed.
- Moderately planted tanks with no shrimp: liquid carbon is usually enough.
- Densely planted, aquascaping-style tanks: pressurised CO2 becomes necessary to keep algae from taking over, alongside a well-chosen selection of live plants.
Find all our planted-tank equipment comparisons on the fishkeeping hub.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.