Testeur d'eau SeaTest PO4 SeaTest PO4 water tester, description and features: Phosphate PO4 is a residue from the breakdown of organic matter such as food scraps, fish droppings, etc.... It can cause algae proliferation. Although it is preferable to keep PO43- values as low as possible, values of up to 0.5 mg/l are tolerable....