·     By using unfiltered tap water for making up sea salt mixes or as top-off water.

• Through many common aquarium products used, such as some sea salt mixes, activated carbon, KH buffers, foods put into the tank, and many other sources.
  Phosphates are introduced into your aquarium in various ways, such as:

Testing & Removing Phosphates:
The optimal phosphate level, especially for reef tanks, is an immeasurable one, with 0.05ppm (mg/l) being acceptable, and an upper level of 0.1ppm. Having a good, reliable phosphate test kit is important. Reef Warehouse has found that Hach and LaMotte brands as being good choices. Sailfert seems to have a good, precise phosphate test as well, but from our understanding the Sea Chem scale measures too high. (Read Test Kits Reviews & Compare Prices)