I used the Red Sea pro kit and it was always clear because my PO4 was at or near zero. Got a Hanna ULR checker and could see PO4 at at a much lower detection level than the RS kits. 6 months of at or near zero PO4 became a dinoflagellates problem for me. So now I dose PO4 to get to 0.1 and the RS kit can detect this level.