I guessed the stains on the bottom and sides were iron, so I tossed in some Vitamin C tabs and sure enough, they each created a little blue circle. Emboldened, I bought a pound of ascorbic acid from the local nursery (only source I could find). Put it in trying to follow mbar's directions, but nothing happened. The pool store sells a "stain remover" for $50 a pound. Is that somehow better than the ascorbic acid (half the price). What should I do? Buy vitamin C tabs and crush them? Get the stain remover? Drown myself?