What would cause ORP to drop from 370 to 30 in a few hours?

[marinetankdiver]

Yesterday morning my ORP was reading 377, it has been in the 370's for as long as I can remember, I have my skimmer draw outside air. Then yesterday afternoon it read 30. All other parameters are fine. Temp: 77, Salinity 33.1, PH 8.01, Nitrates, Nitrites and Ammonia are close to 0 as my test kits show, Dkh is at about 9. All of the corals are extended and looking great as are the fish and inverts. I re-calibrated the probe this morning and it is now reading 70. I'm hoping the only explanation for this type of change without any noticeable effect on livestock is a bad probe but thought I would ask for opinions.

Thanks.

[GlassReef]

Sounds like a bad probe to me - 30mV is way low! If you can get a hold of some 400mV calibration solution, stick your probe in some fresh salt water for a couple of hours. If it doesn't settle at approx. 190 to 220mV its the probe.

[marinetankdiver]

Thanks,

Moved this question to the Chemistry Forum