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 likely 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).
Unless you are an advanced aquarist maintaining a complicated reef system you can usually take care of trace elements with regular water changes. Dosing them is often problematic and unnecessary, and testing for trace elements can be expensive. If you do look into testing and potentially dosing trace elements, Potassium is considered the most important trace element in reef aquariums and likely the best place to start.