Here are some observations that I have noticed over the years with stains. Make sure you add enough sequestering agent. It won't hurt anything if you put more in than you need, but it will leave some metals free to stain if you don't. Keep your ph on the low side - I like to keep mine between 7.2 - 7.4, no higher. Keep you chlorine consistant - don't let it drop down to where you will have to shock. High chlorine levels will let any metals that are free in your pool to precipitate out and land on the pool. When you start to see the stain forming, add more sequestering agent and bring your ph back down to 7.0 - 7.2., and keep the filter running 24/7. It seems that the stain will lift with the sequestering agent and low ph if it hasn't become too embedded. If the stain is localized, put a little ascorbic acid in a stocking and rub it on the stain - this way you don't have to do the whole treatment.