If your plants are fine, then you don't need it. If you want to grow a carpet, and you don't want to wait months for it to cover, or if your plants aren't doing very well then yes, get CO2. I'd lower my CO2 when my plants have grown to where I want them, so that I don't have to trim them as much. Remember that with CO2, you'd need more fertilizers too, depending on how strong your light is.
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