PlantGuy's Mono Potassium Phosphate -KH2PO4

KH2PO4 – Mono Potassium phosphate: Provides Phosphates (P) and Potassium (K) for feeding aquatic plants.

To make a typical stock solution add 15g of dry KH2PO4 to 500ml of RO or distilled water and then add 5ml of this per 100L of tank. Dose 3 times a week.

This will give you 1ppm (1 mg/l).