The reason you are testing no metals in the water is because the sequesterant -pool magnet, is holding the metals in suspension. THere is no way I am told to get the metals out of the water once they are in there, except by a very complicated way that Pool Doc had talked about. I will explain how staining occurs. When there are metals in the water, and you have your ph above 7.5 and get your chlorine levels up high as when you shock the pool, it can casue the metal to precipitate out of the water. This can cause the pool surface to stain. It also causes the pool water to appear light green, or brown. What I do when I open the pool is to add sequesterant first before any chemicals. Leave the ph about 7 to 7.2, and slowly raise the chlorine. If you have a sand or DE filter you can put a trichlor puck in the skimmer basket (this will help by the water going over the puck, if the metals precipitate out they will stain the filter instead of the surface of the pool) keeping the filter running 24/7. Do not do this if you have a cartridge filter. I also had success when my pool was cloudy after a stain treatment, putting in a flocking agent. I let the water sit for 24 hours without the filter running, then vacuumed to waste. I shut the filter off everynight and vacuumed to waste in the morning. I did this till there wasn't any stuff on the bottom, and the water was clear. Whenever I added fill water, I added it through the skimmer with a puck in it. When ever I have to shock, I make sure my ph is down around 7.2. I try to keep my chlorine level constant by testing each evening and adding enough chlorine to keep it in the perameters of the "best guess chart", this way I very rarely have to shock the water. Hope this helps, if you have any questions feel free to ask.