My pool had 1 yellowish stain (was there when I bought the house).....UNTIL my pool guy recommended I get PoolRX. He swears by it. So, I did and next thing I see a couple weeks later are rust stains coming through. I appears to be rebar stains, you can see the rebar pattern. And it has cropped up in various other places as well. I have to assume there is something in the PoolRX that reacted with something. I have lived here for 5 years, no rust spots. The only thing that changed was the addition of this product. So, soon after I removed the product, I visit my son and notice rust stains his pool. He stated, Ya ever since I started using PoolRX my pool started popping up with rust spots so he pulled it out. I have read that there are claims that this product does not cause rust stains. Well, okay, then both our pools all of a sudden broke out in a rash on their own in two different cities with different water compositions. There is no metal, bobby pins, fertilizer in our pool to explain this. Just be aware.