Zapins

True what you say about low light tanks not being able to use the CO2. The limiting factor in most new tanks is light intensity. However, since you already have the CO2 system, it won't hurt adding it on the tank in fact it will probably help somewhat, though not as much as if you had good lights and CO2...

By the way, there is no complete cycle in a planted aquarium. For there to be a cycle the nitrogen must actually be converted into all the sub forms and end up as NO3, however, plants prefer NH3 and will absorb it before it gets converted to NO2 and NO3 so the nitrogen cycle that is so crucial to achieve in fish-only tanks never actually forms in a planted tank. This is one of the reasons plants are beneficial to fish.