Eh my tank is almost 2.5 years old and its a 29g nano, not a large tank and not a ton of rock...oh and its bare bottom, no sand. I have a small amount of sand in the sump for diversity but not a lot. So I do not think that explains it, if I dose I can keep it up but if I stop it goes to 0. I so have a bunch of ceramic rings in the sump and like I said some halimedia macro but just no idea whats soaking up the po4 and I think that may be part of my long standing issue.If your tank is new, most of that PO4 is getting bound to rocks and sand. The rest gets used up quickly by all the life in the tank.