It is very important to prevent phosphate from accumulating in a reef tank since high levels will stress out corals and invertebrates and feed the growth of unwanted algae. Corals suffer because high phosphate levels inhibit calcification but some phosphate is required for growth so stripping out all phosphate is also not good. Given this fact, I like to keep a “slightly dirty” tank and strive to maintain phosphates within the .03 to .07 ppm range. The same is true for nitrates. I keep them low (~ 2.5 ppm) since high levels can potentially be detrimental to coral growth.
There are several ways to control and remove phosphate in a reef tank. A protein skimmer is one tool for removal so it is important to have one that is reliable. Regular water changes is another means for exporting phosphates. I like to change out 10% of my tank water every week. A third method is to set up a refugium, algae reactor or algae turf scrubber and grow macroalgae, a natural method for reducing phosphates.