here is a quick before and after shot of the chemical i just mentioned. This is a filter at a city water plant. The trough on the left was left as it was found and the one on the right was sprayed with the chemical, allowed to sit for a few minutes and rinsed with a fire hose (simply for volume, not pressure) and had the filter running back online in about an hour after the backwash. It does the same thing to the stains on a boat hull. The water in these filters is just lake water with chlorine added to it before its filtered so its the same stains we all get.
Heres a quick video of the procedure, its boring, trust me but give it a couple of minutes and wait til we start spraying.