Ozone is an air pollutant. It is irritative to both human and fish.
But I believe that the dose of ozone that can be generated by an ordinary hobby equipment is not high enough to cause real harm. With suitable caution, the risk of using ozoniser is very small. I have been using ozoniser for more than one year. Some leakage of ozone do occur due to break down in the tubing. But it had not caused any real damage to me or my family. I will continue to use ozoniser.