If all your tests are a-ok, especially no ammo or nitrites and low trates, go ahead and add a few things now. As long as you dont have any puffers or trigger fish-the snailcrunchers, you can add about a dozen each of nassarius and turbo/astrea snails.
Again, depending on what else you have in your tank, you can add a star, cucumber, or some shrimp-not all at once though, but some every week should keep you busy for a while!
What fish do you have in there now?