Seachem Flourish Phosphorus is a safe solution (4,500 mg/L phosphate) of potassium phosphate that takes the guess work out of phosphate dosing.
Features: Phosphorus supplement for your planted aquarium, A safe 4500 mg/L solution of potassium phosphate
Directions: Beginner Use 2.5mL (half a cap) for each 80L (20 US gallons) once or twice a week or as needed in response to signs of phosphorus deficiency (e.g. stunted growth, plant dark green).
Expert: The beginner dose raises phosphorus by 0.05 mg/L (0.15 mg/L phosphate). The ideal phosphate level will vary, but generally ranges from 0.15-1.0 mg/L. To target a specific phosphate increase, dose according to the following formula: 0.8vp=m, where v= volume of tank in gallons, p=desired phosphate increase and m=volume of product to use in mL. For example to raise 20 gallons to 0.1 mg/L phosphate you would use: 0.8*20*0.1=1.6 mL.