I'm testing my pH using the API liquid test kit. When I lower the test tube into the water, the water cascades in a thin film down the sides of the tube and a precipitate forms becoming a solid that floats in the water. Any idea what is going on?? The water is crystal clear, doesn't have an oily film on the top and yet it only forms in the test tube. I'm dosing PPS-Pro, could it be chemical residue?