When do I need CO2 equipment?

If they're growing well and you have to trim them, then why do you need CO2 at all?

IMHO you should only use CO2 if you need to use it. If your plants are already nice and healthy, why bother? Just sit back and enjoy the plants!

CO2 equipement is generally (though not exclusively) reserved for people who have higher amounts of light on their tanks. If using normal output flourescent lighting, with no other light sources on the tank (ie. ambient lighting, windows, etc.) then 2 watts/gallon is the general borderline for using CO2. Any less, and CO2 is generally not needed... any more, and CO2 is generally needed. Once you factor in, other sources of light, the borderline can be shifted down to lower watts/gallon rating. Using other sources of light, like power compacts (aka. compact flourescents), or metal halides can also skew this general rule of thumb. Like many things in this hobby, it all "depends," almost nothing is written in stone.

If your tank is doing well, with little/no algae growth, then it will do fine without CO2. However, if you want to further inhance your plant growth, adding CO2 should help... you can add that whenever you want. I know of a few people who supplement their low light tanks with CO2 injection.
Now if you're planning a high light tank right from the start, then you need to be able to get CO2 at the right levels right at the start... before you add any plants to the tank. Or if you plan on increasing the amount of light on your tank, then make sure you get CO2 injection in first, before the new lights. In a nutshell, if given the choice between upgrading your lights, or getting a good CO2 system.... go for the CO2.

HTH
-Richer