Does your ozone generator have any adjustability? That's the only way I know how to operate a 200Mg/hr ozone generator at less than full capacity.
If not, the only way to control the amount of ozone that gets injected into your aquarium is to set the ORP controller at the desired point (hopefully somewhere between 300-400mV). That way, the controller will prevent the generator from producing too much ozone for the tank and shut it off when the ORP reaches the point you set.
The only way I know of to reduce the amount of ozone coming out of a generator is to increase the humidity of the air that the generator is drawing in. Higher humidity levels reduce the generator's effectiveness at producing ozone.
Also, watch your pH levels closely once you start administering ozone. When I first started using it, my pH level dropped as the ORP climbed (it was 250mV before I started ozone and the pH dropped to 7.75). Once the ORP reached 375mV (after a few days), the pH stabilized back to the low 8's. Now, I only notice the ORP react during a pH spike (it drops about 10-20mV when I add buffer on occasion).
