co2

[lee8040]

what is it ?

 

[nodding_dino]

Carbon Dioxide

 

[IMCL85]

something that u need in heavily planted tank with strong lighting applyed.

 

[freddyk]

It's also something you don't want in a tank without plants to use it. Co2 is much more easily absorbed then oxygen, something easily countered with good water and surface movement.