When you say "leave it", I assume you aren't familiar with the cycling process, or don't understand how to fishless cycle a tank? Simply keeping water in the tank for a few days won't do a thing. You must have an ammonia source. I recommend fishless cycling with ammonia found at the store...of course, making sure it has no added dyes or parfumes in it.
To read up on carbon, go to the useful links thread in this forum where we have included many good links to cycling an aquarium.