I put a few ounces of this every week just like using shock and it keeps my vinyl perfectly clean.This stuff is fantastic. We have copper in our water so I always use the "treated water" to fill my 18x42 inch intex pool. We must have run out of salt and some copper bled through. When I threw the chlorine in a green stain attached to pool bottom it would not rub off. Looked terrible. I threw in half a bottle my pool is 5000 gallons. 2 hours later the floor was back to its original condition and there were drifts...like of copper dust. I stirred it up and ran the filter...it all came out into the filter and my pool looks great.Seemed to workI have been using this for years. To start with, I have a fiberglass-lined concrete pool. Fiberglass may be easier to remove stains from than concrete but it has done a good job on removing the "normal" stains" that occur for over 5 years. This year, however, my pool "went south" when I was on a 2+ week vacation. It got green algae and some MAJOR staining.Blue Stuff removed about 80%-85% of the stains. I spoke with the company the tech was a little surprised it worked so well, because apparently it ISN'T made specifically for stains However, when I mentioned that the pool is fiberglass-lined, he said that that may be the reason it removed most of the stains.Based on my results with this product, in the fall I will buy Jack's Magic's stain test kit, analyze what kind of stains are remaining, and buy what they recommend. It can be difficult to find products that REALLY work, Jack's Magic does! Great company and great products. (FYI, Apparently Leslie's pool supply has discontinued it!!???)I use this in the fall. The leaves from some of the trees over our pool carry iron into the water and stain the pool. (Jack's confirmed that this can happen. This is in addition to stains from tannins, which chlorine can handle.)Adding some Blue Stuff each week keeps the iron suspended and off the walls. Note that you will need a DE or Zeolite filter to physically remove the suspended iron from the water. You can also add DE or fiber to a sand filter. If you don't filter out the suspended iron it will just precipitate back out.I guess it works. Pool people recommended it and they do not have any complaints.This is a great price over what pool stores charge for it. Arrived on time and I will be a repeat customer.We have a 25,000 gallon inground salt water pool with a vinyl liner. Last year we had a stain develop on the one of the side and It did not matter what we used the stain would not lift. When we opened the pool this year the same stain was EVERYWHERE! I purchased one bottle of this and poured it around the pool on one side to see if it even worked.... at first I was disappointed because I thought it was going to be instant.. went to sleep that night and worked all day came home and looked at it and every stain on the liner is GONE!!! I am so pleased with this product !!!