Filtration, both "natural" and artificial, is the essential element that makes our closed recirculating systems possible. Keeping fishes comfortable, healthy, long-lived and even productive in their isolated synthetic ecosystem usually doesn't require adding ingredients to the water so much as it involves taking some out. For instance, some "natural" bio-filtration is performed by plants, which you periodically export from the system.
Filtration, in its three manifestations, is a broad subject that I'm not equipped to cover in full, but I'm urging you to think it out in detail before you begin setting up. In this "Filtration" folder you'll find material on the concepts involved. The actual filtering equipment used will eventually be in the "Setting Up" folder.