With 100% certainty, I don’t. I normally dose 1-2mL (solution is 10mL per 1/2 gallon) at a time and that will bring my phos down by roughly .04. Due to a Fusion error once, I accidentally overdosed...about 12mL. Thought I was going to kill everything in the tank, but evidently the skimmer caught everything. No ill effects to fish or coral.