Can you explain this Redox issue?
 

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.

The ORP drop might have been caused by a number of problems. Something large in the live rock might have died, a coral might have spawned, or some other organic matter might have gotten into the system, just as a few example.

Personally, I wouldn't worry so much about the current ORP. The corals might perk up in a bit. I'd keep up the carbon.

It might also have been some issue with the probe itself. Did you check it visually? Maybe a snail was on it?

Thanks for your responses. I am fairly certain the probe is OK. First of all it was the condition of the corals that made me look for a cause and the only apparent problem was ORP which had dropped by 200 mV. The probe is floating in the return area of my sump/fuge and it was clean and unobstructed. Also a check a 400 mV cal. solution came out to very close to 400 mV. Now several days later, ORP seems to have stabilized at 290-325 but is not improving (nor getting worse). Corals are OK but my Xenia is less "plump" looking and my star polyp colony is only about 85% fully open. They all look good but not terrific as they did before. Still puzzled but there seems to be no immediate problem.

Maybe something disturbed a patch of sand, releasing reducing (low ORP) compounds into the water.

Randy, that sounds like a reasonable possibility. I have about 2 inches of sand a substrate, mostly covered by LR. Is it common for the live sand to start contaminating the system after a period of time?

Come to think of it, according to my log, I treated the system with Red Slime Remover to treat a cyano outbreak that I was able to stop but not eliminate with new PhosBan, multiple water changes, water flow and skimming. That was about a week before the ORP reduction. Could the RS Remover have killed the sand allowing it to release ORP reducing compounds? Problem is, here we are several days later and ORP seems to have stabilized in the 300 range, neither improving or worsening.

People have reported sandbeds that released nutrients, and that possibility seems reasonable. Whether or not that's happening in your system is hard to say.

Johnny, ORP is a strange beast.

You were very fortunate to have an orp of 400 without running ozone. And I'm not sure this means anything in the health of your tank.

Many things affect ORP and I would not rely on ORP to tell you the health of your tank. It can be very misleading to aquarists.

I've been dabbling with ORP and ozone for a few years now and there is no rhyme or reason to the numbers. Period.

I use an ORP meter to monitor my ozone injection.

If you are not using ozone, my recommendation would be to put your ORP meter in the drawer or sell it to someone who needs it.

You don't. :D

If you are seeing some distress in your corals, I would look to another reason.