Check this out I have a 75 gallon SPS system, I run ozone faithfully when my ORP drops below 350 mind you I have 6 med to small fish. I have a friend who runs no ozone... water slightly yellow no where near as clear as mine, he has 18 fish small maybe a couple med size... does water changes sporadically at best, his orp runs 370-390!!!!! Now I have checked both systems with 3 different types of ORP controllers all read the same. Is this possible?? I am completely confused with no answer!! Any ideas???