Well, what kind of tank do you have? soft coral tanks, LPS tanks and mixed reef tanks don't really have to worry about depleting phosphate levels. If you have a phosphate problem, you will see excessive algae growth. If you have an SPS dominated system and oversized equipment, you may want to make sure you have at minimum a 0.03ppm phosphate reading or else you risk bleaching your corals. Really, you don't *need* it, but it is a good tool to use if you have a higher-end SPS dominated reef.