I have the HI774 Ultra Low Range Phosphate checker and it always says my tank is at 0.01Think I got my posts mixed up, you have the 736? The phosphorus checker, it has a -/+ of 10ppb which is .03 so is more than fine for your tank, just aim for .04 or above so you are always above zero.aiming for .04 means your tank could be anywhere from .01 to .07 if the reading is showing 0.04.