I get anywhere between 300 and 400.
Also higher the pH lower the ORP will be.
You can get it up by injecting ozone into your skimmer.
But mostly you just watch for changes.
Last thing, dirtier the probe, higher the reading and after cleaning it really goes down for a few hours.
It's also good to have a ground probe in the tank.
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Denis
150g SPS reef;3x250WHQI,Tunze Streams,Wavebox,Neptune Apex,fishroom in basement  ,reefing since 93,SPS since 99.
TOTM June 2010
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