KH is probably the single most important ion to monitor in SW aquariums. Let's put it this way, Ca, Mg, K+ too low/high? Animals become unhappy. kH out of whack? Everything dies.
Pick up a test kit for kH; I'm partial to the Salifert one. Easy to use, relatively inexpensive. You'll want a firm grip on what kH is and how it interacts with other major ions. Here is an article to get your started:
http://reefkeeping.com/issues/2004-05/rhf/