I don't generally recommend a target level, just a level to not exceed. I discuss such issues in these articles:
Ozone and the Reef Aquarium, Part 1: Chemistry and Biochemistry
http://reefkeeping.com/issues/2006-03/rhf/index.php
Ozone and the Reef Aquarium, Part 2: Equipment and Safety
http://reefkeeping.com/issues/2006-04/rhf/index.php
Ozone and the Reef Aquarium, Part 3: Changes in a Reef Aquarium upon Initiating Ozone
http://reefkeeping.com/issues/2006-05/rhf/index.php
ORP and the Reef Aquarium
http://www.reefkeeping.com/issues/2003-12/rhf/feature/index.php
from the second one:
So with all that background discussion behind us, here are my recommendations for ORP monitoring and ozone control in reef aquaria using a properly sized ozone generator that appears to be working, and a properly calibrated ORP meter:
1. If the ORP never seems to rise above 375 mV after initiating ozone, do not worry about controlling the ozone or the ORP. Just let it run full out. Also, do not worry about needing a larger generator, assuming it has driven up the ORP by at least 25 mV above where it was before adding ozone. It is likely accomplishing the necessary tasks (such as making the water clearer). Only if some other aspect of ozone use is unsatisfying (e.g., lack of water clarity) would I look for other options such as a larger ozone generator or a better contact chamber.
2. If the ORP starts above 375 mV, or rises there during ozone use, using an ORP controller would be valuable to prevent the ORP from rising too high. Use the controller to shut off the ozone when the ORP rises too high. Another option would be to shut off the air flow to save the dryer's media, but be sure that water cannot flow back into the ozone generator if the air stops. I would set the ORP target somewhat above the baseline ORP in the absence of ozone - at least 350 mV, maybe 400 mV, but never above 450 mV.
