Natural seawater phosphate levels range from under 0.01 mg/L to 0.3 mg/L, with reef aquaria ideally kept at 0.2 mg/L or less for corals. While generally non-toxic to fish and invertebrates, phosphates should be below 1 mg/L to control algae. The MultiTest™ Phosphate kit accurately measures inorganic phosphate down to less than 0.05 mg/L in both marine and freshwater, though freshwater samples often require dilution due to higher concentrations.