I am also using O3, my tank is 70" long of total approx. 800L. I use Sander C300. I found after using the O3 for certain period of time. The water quality can be improve with my ORP reading in between 320mV to 350mV. I turn on 8 to 9 hrs per day when no one at home. Shut down after work and open all the windows for 15 mins to get rid of any O3 remains indoor.
But it can not cure white spot or any other fugus problems even I turn it to 60% fire power and I experienced the O3 kill my fish before. As per LC mentioned. To cure white spot or fugus, I added copper and TDC to gain max.