The short answer
For most aquariums, no — you don’t need CO2. Injected CO2 is only necessary for demanding, high-tech planted tanks where you’re pushing fast growth and difficult carpeting plants under strong light. A fish-only tank, or a planted tank with easy species, grows perfectly well without it. CO2 is an enthusiast’s tool, not a beginner requirement.
When CO2 is worth it
Plants need carbon to grow, and CO2 is their main source. In a high-tech setup — bright lighting, rich substrate, regular fertiliser dosing, and demanding plants like carpeting species or red stems — the small amount of CO2 in the water becomes the limiting factor. Adding pressurised CO2 unlocks lush, rapid growth.
You’d want CO2 if you’re aiming for:
- A dense carpet (dwarf hairgrass, Monte Carlo).
- Fast, vivid growth and bright reds.
- A competitive aquascape.
When you can skip it
Skip CO2 entirely if you have:
- A fish-only tank — plants aren’t the point.
- Easy, low-light plants — Anubias, Java fern, mosses, Cryptocoryne, Vallisneria all thrive on the CO2 fish already produce.
- A low-tech planted tank — modest light plus root tabs grows a beautiful jungle with zero CO2 gear.
Most beautiful planted tanks you see online are actually low-tech. You lose nothing by starting here, and easy plants are far more forgiving.
If you do want to go high-tech
CO2 adds cost, maintenance and a bit of risk (overdosing stresses fish), so go in with your eyes open. Turn it off at night, dose gradually, and use a drop checker to monitor levels.
Start with CO2 for beginners, browse the CO2 systems hub, and see our best CO2 system picks. Getting the balance right also means dialling in your lighting schedule.