Watching some program about housekeeping one day I found out substances like vinegar and bicarb of soda are good for getting rid of the limescale in your kettle and stuff..hardwater stains on your tank are the same thing. I think you would have to mix up a solution of water, bicarb of soda and vinegar (clear vinegar preferably), I'm not sure what the ratio would be only that water forms the biggest part of the solution. I'm guessing you would dip a clean cloth in the stuff and start scrubbing at those stains. Don't recommend trying that on an already setup tank though as it might hurt the occupants. I'd try it on an empty tank, will probably take some really good scrubbing though and once the stains are off you'd have to give your tank a very thorough rinse clean before you can put anything in it.