My suggestion is Oxalic Acid. It's a powder - a couple of tablespoons, diluted in a gallon of water. It is also amazing for that brown stain you sometimes get below the waterline. As with other acids, keep it off aluminum trailers, etc. (tends to 'spot' the aluminum).
I only find it at Ace or other "real" hardware stores...never seen it at Lowe's or the other big boxes.