Any amount that you can smell can do significant oxidative damage. Ozone is very dangerous to your lungs and eyes and respiratory passages, even at very low concentrations. Used properly, an ozonizer should be safe. However, they can be a fire hazard if they malfunction, in addition to being a health hazard. Personally, I would never use one for these reasons, even if they were good for the aquarium. Many people (including me) believe even a properly functioning system does more harm than good.