Water sample returned no minerals in pool water. Chlorine, TA,calcium and PH perfect. This pool was newly plastered about 6 months ago. I am seeing brown staining on all the walls and steps. This is an indoor pool. What could be causing this? It's just getting worse. I tried pouring muriatic acid on a step in the water and it reacted very quickly like a puff of smoke. Stain completely gone.