[Anselth]

JT, simple supply and demand. Consider this analogy: An animal locked in an airtight room will eventually use up the oxygen in the room through respiration. If you check the room after the animal has expired, you will find negligible levels of oxygen. Doesn't mean it wasn't there. Algae are very good at assimilating nutrients that they need to survive. You're not testing your water every second of every day. Unless you're testing right after a feeding, if you have an algae problem, you're likely to get a reading of zero because free phosphates have probably already been assimilated into the algae's tissues. And no, removing all of the algae would not cause an immediate rise in phosphate, the same way that removing the expired animal from our airtight room will not immediately add oxygen to the room. It has been used by the organism for biological processes which fix the ion, removing it permanently from the system.