A7840 Nutrafin Phosphate P04 (0.0 – 1.0mg/L) for Fresh & Salt Water 75 tests – Nutrafin? Phosphate (0.0 – 1.0 mg/L) Test measures phosphate in fresh and saltwater aquariums. Phosphate levels should be monitored weekly. High levels of phosphate, often the result of overfeeding or infrequent water changes, can contribute to unsightly algae growth. Therefore, phosphate should generally never exceed 1 mg/L.