Here are the acceptable ranges for most saltwater aquariums. Temperate aquariums, or those that mimic a specific biotope will differ, but for the most part these are the values you will be using as a goal point:
|Ideal||Reef Tank||Macroalgae Tank||Fish Only|
|Ammonia||0 ppm||0 ppm||0 ppm||0 ppm|
|Nitrite||0 ppm||0 ppm||0 ppm||0 ppm|
|Alkalinity (meq/L)||3.5||2.5-4 meq/L||2.5-4 meq/L||2.5-4 meq/L|
|Alkalinity dKH||10||8-11 dKH||8-11 dKH||8-11 dKH|
|Phosphate||0 ppm||0 ppm||<.5 ppm||<.5 ppm|
When comparing your parameters to the values in the chart above, it is important to remember to plan for changes/corrections only over a period of time, and only if you are not getting the results you want. If it isn't broken, don't fix it. The exception is Ammonia, Nitrite and Nitrates, which if too high can be brought down quickly with water changes with a likley positive benefit. You will want to make sure the water used in the change is close to the other parameters that are present in your tank if you do a very large water change. Of special importance during water changes is pH, Temperature, Salinity (measured here as specific gravity; even though that is not entirely accurate that is what you will likely be using).
At this point you may be wondering, what about Boron? Manganese? Iron? etc... The thing about trace elements is they should be taken care of with regular water changes, and testing for them can get expensive, and dosing them is often problematic or unnecessary. As long as you use a quality salt brand, and are maintining the parameters above, trace elements should fall into place in your tank and you shouldn't have much to worry about in that area.