Seachem Flourish Phosphorus, 500 ml, is one of the main three macro nutrients (nitrogen, phosphorous, potassium: NPK) required by plants and can often become the limiting factor to growth in a flourishing system. Flourish Phosphorus is a safe solution (4,500 mg/L phosphate) of potassium phosphate that takes the guess work out of phosphate dosing. Unlike competing products that mix nitrate and phosphate in a fixed ratio, Flourish Phosphorus (being nitrate free) allows you to dose phosphorus according to the needs of your plants without the risk of overdosing nitrates.