Well this topic alone takes up many chapters in several books that I have, so a brief post will not do it justice. Essentially Ozone is a chemical sterilizer that purifies water. Like other chemical filtration techniques (Ultraviolet, activated carbon), there is a fine line between enough to counteract negative things like polution or parasites and enough to kill all the good things in your tank, including your animals. The measurement used to control dosing of Ozone is Oxidation Reduction Potential (ORP). So if you were to start using Ozone on your tank, you should be prepared to accurately measure ORP, and this is not easy as you need a special meter and probe that can be a pain to calibrate. Typically Ozone is used in tanks with heavy bio-load (big fish, lots of carnivores, etc.) I would not go down the Ozone route unless I already exhausted the easier ways to keep my tank clean like good skimmer, regular water changes, good husbandry.