Don't quote me on this, but lowering the pH level may whiten it, similar to acid washing the surface.
Short of draining and acid washing the surface, there are "no drain acid wash kits".
You can also try to isolate the area and dose some acid over the area.
Since you're in South Florida, so is SGM (they're in Pompano). May be worth a call to them to see how they recommend treating it.
