We have a water softener in which we use potassium salt. We are on city water, not a well. We have been getting a "shot" of blackened water appearing anywhere from 5 to 20 seconds after starting to run our bath and it only lasts about 5 seconds before becoming clear water again. We don't seem to see get it at any other taps but then maybe its just more visible against the light colored bathtub. It seems to be worse in the 1 - 2 days after the softener has regenerated and the water is at its softest. It is also more concentrated in the hot water than the cold. Our plumbing is a combination of copper pipe and flexible hose-pipe. Our house is 50 years old so some of the copper pipe is that old. Any of the flexible pipe is less than 2 years old as is our hot water tank. Is the softener salt leaching something out of our water lines or faucets? Could the water be reacting with the aluminum sacrificial rod in the tank? This problem did not start immediately after installing the softener. It has just recently developed about one and a half years after installing the softener, tank, flexible pipe.