They will absorb until they hit equilibrium with the water. I don't know how much it will take to reach that point. It depends on the volume and the current phosphate level in the rock/sand and difference between water concentration. I would calculate the dose to go to the level you want to target and test it in a few days. Calculate the new dose needed to hit your target level again and then repeat.I ended up getting neonitro as well to get overnight shipping. Ill dose it and up feedings until i see a good nitrate level, then ill back off the dosing. What kind of time frame am i looking at for the rocks to finish absorbing phosphate?