I have reviewed the forgoing string. I see that you are having a problem with water that stains your toilet bowl brown.
Although you may have soft water, the definition of soft water does not pertain to the amount of iron you have in solution in the water. Hardness refers to calcium carbonates in the water.
A lot of water will have iron sulfate (ferrous sulfate) in suspension. When it comes into contact with certain other chemicals, it precipitates out as a brown to reddish color and can leave your porcelain stained.
It may not be coming from rust at all, but he precipitation out of ferrous oxide onto the surface of your toilette bowel when it comes into contact with household bleach, or chlorine that is used to purify your water. Fluoridation can also cause some light precipitate. If your hot water heater is not softened, it will show higher amount. Your water softener would filter out some of the ferrous sulfate, as it would adhere to the soften beads in the presence of salt. You an ask the water softener guy to check your water for iron. You can also get some small home testing kits at the hardware store and test it yourself. Pool supply stores also have these test kits.