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[IslandReefer]

Hey Gary,
Not that I can speak for anyone else, but here's what I monitor:
Salinity-refractometer
Temp-pin point wireless thermometer-cool but expensive...Radio Shack sells a dual digital display for about $25 (Steve uses one)
RO/DI water - TDS pen- anti algea check!
water chemistry: Salifert test kits for dKH, Ca used apx. weekly.
pinpoint pH meter: takes a long time to stablize(-I might try one of the pens next time)
Also when trouble shooting or cycling I have also used Salifert test kits for NO3,NO2,NH4,Mg.
Monitors for pH,ORP etc. are useful or needed for use with a Ca reactor or eg. an ozone generator etc.
IMHO
Hope this helps