Some of you might already know this but its an accidental discovery for me. I have a large cherry tree partially overhanging my pool and this time of the year I get small decorative cherries dropping in the pool and staining my new white plaster as they disintegrate before my pool cleaner gets to them. I had some Cal Hypo and sprinkled some over the spots and the next day all the spots were gone. Since it does not dissolve very quickly it gets to the bottom of the pool and sits on the spots bleaching them.