Aluminum Boat Water Stain Remover

I found one thread on removing scum from a boat hull but nothing on stains.  The lake we're is a stained lake.  Pretty much the color of weak coffee.  My boat that I leave up there has developed a thick brown band at the waterline.  What can I use to remove it?  It's a painted aluminum hull.  My dad uses Zing toilet bowl cleaner to clean his pontoons every fall but those are unpainted. Can that damage the finish on a painted boat?  If I can get away from the boat hull cleaning products with their ridiculous prices that's the way I'd go but not if I'm going to damage my finish.

I have successfully used CLR aka Lime Away on my painted aluminum boat. Spray it on and the scum line wipes right off. It's not recommended for bare aluminum. But it does a great job on the painted stuff. It'll also get rid of the chalky build up on a black lower unit. Hose it down when you're finished.