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: carpets and dense fast growth need it, and hardy epiphytes and rosettes do not.
What you can grow without it
A good deal, and more than most beginners assume.
- Anubias and Java fern, both low light, both no CO2 required.
- Java moss, which is about as forgiving as an aquatic plant gets.
- Cryptocoryne wendtii and Amazon sword, both undemanding rooted plants.
- Floating plants, which sit at the surface with direct access to atmospheric carbon dioxide and grow quickly regardless.
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.
Where it stops being optional
Carpet plants are the dividing line. Dwarf hairgrass will not carpet tightly without added carbon, and dwarf baby tears requires it outright. Without it they grow leggy, lift off the substrate and fail.
Strong colour is the other case. Rotala rotundifolia only reddens properly under high light with added CO2, and stays green below that.
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.
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.