I think the nature of the animal is such that these things dissolve quickly. If they were too hard, they would scratch the surface. I've used all six on about 12 foot pool tiles. I had some kind of black mark (I'm the second owner of the pool) coming from somewhere and I wanted it to go away. Removed how. It's not new yet, clean and shiny, but it's getting better. A word of caution: remember that all that tiny pumice will mix with the water and fall to the bottom of the pool. 6 weeks later I am still cleaning pumice with my floor cleaning robot and I think it will probably stay in my pool forever so take that into account. I will try hydrochloric acid to remove the remaining stains although I bought more pumice. Now I'm going to use a pumice stone to remove hard water residue from the toilet.
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