No, you do not need CO2 for a planted tank. Plenty of hardy species grow well without it, and an entire method is built on going without. What CO2 changes is which plants are open to you: Tropica rates the finest carpets, such as HC Cuba and Glossostigma, as high CO2 demand, while hardy epiphytes and rosettes are rated low.
What you can grow without it
A good deal, and more than most beginners assume.
- Anubias and Java fern. Tropica rates Java fern low for light and CO2, and Flowgrow says Anubias grows with or without added CO2.
- Java moss, which Tropica calls a hardy plant that makes few demands on the water or light.
- Cryptocoryne wendtii and Amazon sword, both undemanding rooted plants.
- Floating plants such as Amazon frogbit, rated low for light and CO2.
That list will fill a tank and look like a planted aquarium. The Walstad method and the jungle style are both designed around exactly these species. To check any plant’s CO2 rating, filter the plant ratings chart.
Where it stops being optional
The demanding carpet plants are the dividing line. Tropica rates dwarf baby tears and Glossostigma as high CO2 demand, and says Monte Carlo needs added CO2 to become really compact. Not every carpet is on that side: dwarf hairgrass is rated low for both light and CO2.
Strong colour is the other case. Tropica says Alternanthera reineckii needs CO2, along with good light and fertiliser, for growth and colour development. Rotala rotundifolia is different: its CO2 demand is rated low, and good light is what brings out its red leaves.
The catch nobody mentions
Going without CO2 is fine. Running strong light without it is not.
Most aquarium plants do not thrive without CO2 fertilisation, but algae does. Light drives demand for carbon, so a powerful fixture over a tank that cannot supply it hands the advantage to the organisms that cope better with the shortage. That is the mechanism behind black beard algae, which takes carbon from hydrogen carbonate far more easily than higher plants can. The hair algae page covers the other common result.
So the honest answer is not simply “you do not need CO2”. It is that low-tech means low light as well, and the two go together. Match your lighting to the plants you are actually growing and a no-CO2 tank is stable and easy. Overlight it and you have bought yourself an algae problem instead of a shortcut.
Related questions
Is liquid carbon a substitute for CO2 injection?
Not an equivalent one. Liquid carbon products are widely used and also work against some algae, but they do not replace injected CO2 for growing a demanding carpet. Treat them as a supplement rather than a swap.
Will my plants die without CO2?
The hardy ones will not. Demanding species chosen for a high-tech tank will struggle, grow leggy and eventually fail. The problem is nearly always the plant choice rather than the tank.
Should a beginner start with CO2?
Usually not. Learn how plants behave on something forgiving first. CO2 injection adds cost and one more variable to get wrong, and it is easier to add later than to unpick a tank you set up badly.
Sources: Tropica, CO2 plant care guide; Tropica, Microsorum pteropus (Java fern); Flowgrow, Anubias barteri var. nana; Tropica, Taxiphyllum barbieri 'Bogor Moss'; Tropica, Limnobium laevigatum; Tropica, Eleocharis acicularis; Tropica, Rotala rotundifolia; Tropica, Micranthemum tweediei 'Monte Carlo'; Tropica, Micranthemum callitrichoides 'Cuba'; Tropica, Glossostigma elatinoides; Tropica, Alternanthera reineckii 'Rosanervig'; Aquasabi, Black beard algae (BBA).