Can you explain this Redox issue?

[JohnnyM2]

My 90 gal reef tank has been running one year and I have taken pains to maintain water quality as perfect as I know how: RO/DI water, 4 ea. 650-850 gal/hr power heads (SEIO and Koaralia), chiller and heater set to maintain 77 deg. +/- 1 deg, Big skimmer (Trigger Systems 150), sump, calcium reactor, top off doses Kalk, phosphate reactor. All of this is controlled by a Neptune Aquacontroller to switch lamps, pumps, Ca reactor, etc. Calcium, Kh, pH, ORP, Nitrates, phosphates etc. are regularly monitored and all have been within my established range with few excursions outside that range. Recently my ORP, which had been at close to 400 for a very long time dropped precipitously (I would say over around 48 hours) to 173. I noticed this because my softies suddenly looked really poorly. With the possible exception of one or two small snails (hard to keep track of) nothing had died in the tank. After several large water changes, addition of charcoal, turning on UV, ORP returned over several days to around 300 but won't go higher. Water changes cause a small ORP drop (understandable). Totally dismanteling and cleaning the skimmer did not help even after 2 days. I check the ORP probe and it reads correctly in cal. solution. Corals have almost completely recovered but I don't understand the change. Does anyone have any other ideas? BTW I'm not set up for ozone and not a fan of it for rounine maintenance due to the risks and my sense (perhaps unfair) that its a crutch to allow other things to be done wrong. It should be possible to maintain ideal water quality without it, I think. Anyway, I wold like to be able to get back a peak daily ORP of 350 and a minimum of 300 (currently 285 to 320) because I have a sense that my corals don't look quite as good as they did previously and I want to start adding SPS corals which are more finicky.