I tried using Salifert for p04 and no3 for a while. I found my API test kit better for no3 but still not great. I found it very hard to see a difference in color on the Salifert ones. I ended up buying a Hanna ultra low range phosphorus checker (was only $30 on Amazon for a bit) and the Red Sea Nitrate Pro kit.Just got a sailfert po4 tester. It reads zero. Anyone use sailfert tester for po4.