It'll clean stains from the inside of your toilet bowl, just like it cleans false teeth (yes, it does work - not wonderfully, but it will work to an extent, if you drop 4 tablets into the toilet bowl and leave them in overnight - provided that there's water in the bowl). I have serious doubts however, that it'll have any effect whatsoever on the inside of the water tank. I suspect there was a bit of a misunderstanding on someone's part.