I dose my 120g tank with Flourish Iron, among other things, and test constantly to monitor my parameters. For some reason, iron will never stay in the system; 12 hours after putting in 30 ml (twice the recommended dosage), my iron test shows the level to be 0.0 ppm). So I put in 30 ml more, and the next day the level is 0 again. Could my plants possibly be using up all this iron? I'm keeping amazon swords and some java moss. I know the test kit works, because the iron shows up if I test immediately after dosing. My substrate is mostly gravel, with some fluorite mixed in. I have a bunch of driftwood in there, no carbon, a little green spot algae on the glass. What's going on? Should I continue dosing up to 60 ml of Flourish Iron per day?