Hi victor90,
For a 90 aga, consider about 30 mg/hr piped into the protein skimmer (It doesn't take much at all to get the water clear). Use an ORP controller and shoot for 350. Its a good idea to run your return water through a carbon filter.
As I said before, I dont really look at my ORP or run carbon. The above is the proper way to use O3. I look at my tank and decide if the water needs more O3. Otherwise, I run the lowest amount possible.
Brad