Elevated phosphate levels in the aquarium are obviously a bad thing, but for more reasons than you think. Besides fueling algae growth, phosphate is also known to slow coral growth as it prevents the calcification process in many species. Another issue with phosphate is that it is fairly hard to test for, and even if a test shows zero, it doesn’t mean your tank is phosphate free. Algae take up phosphate at such a rapid rate that your water may test “clean”, but in actuality the test results are masking some major issues in nutrient control. To keep phosphate levels in check, we recommend using granular ferric oxide (GFO) media and performing regular water changes with good source water.