Redox levels

[niloc16]

i just calibrated and connected my redox probe for the profilux and initially i had a reading of 228mv and now it is down to 202mv. i've been having a hell of a time researching redox and its relation to the tank. from what i have found it should be reading 400 + or - some for natural seawater. how do i raise the redox level? any good web pages as to researching redox?