People who visit my living room are often struck by the sheer, jungle-like lushness of the vegetation in my 125-gallon aquarium. The tank has such a profusion of plant life that its fish sometimes fight for the clear spaces or disappear for weeks on end in the thickets, living as they would in only the most abundant natural settings. This is a far cry from the aquaria I maintained as a child, when the only plants I could keep alive were the most beginner-friendly, least demanding species, if even then. Perseverance got me to my current skill, and a key part of that perseverance is learning my way around more advanced tools of the aquarist trade. And for someone who takes great joy in aquatic plants, that means carbon dioxide (CO2).