Oxygen
Redux
Potential
Not something that's worth worrying about in a marine aquarium IMO because 1. test kits and/or monitors are basically unreliable and 2. it's not something that's easily changed and 3. If the tank is set up properly, it will take care of itself.
Check out this link:
http://reefkeeping.com/issues/2004-05/rhf/index.htm