Beth...wrong...If the water tests zero, it only means that the available inorganic phosphate inputs (foods, etc) are being locked up organically by the algaes,sand bed, liverock, etc. Phosphate doesn't last long freely in the water ...all the bacteria and algaes want it....the increase in total phosphate in the system (locked and unlocked) is a hard thing to test for since our test kits can only detect what's in the water (unlocked). The increase in the total amount of phosphate usually manifests itself as increased hair algae and cyano.....again, this why some tanks have a hair algae problem and still test zero for phosphates So, in essence, you are correct in saying that "it is balanced" except that this balance is created by an increase in algaes or a decrease in the amount of phosphate holding ability of the sand and liverock. If the phoshate isn't exported by some means, the system, some day, will no longer be able to hold anymore phosphate and begin to release it back into the system. So, if you can detect phosphates, you already have reached the system's capacity... I only use my phosphate test kit (Sailfert) to measure things for phosphate leaching that I want to add to the tank... like carbon...the tank water will always register zero...or at least I hope so.
Here's an interesting tip that some folks are doing to test their sand beds... they take a water sample right near or slightly below the sand surface and test that to see if the sand bed is leaching phosphates...interesting, I thought.
