I had a Kohler Devonshire whirlpool tub installed. When used with
jets turned on a yellow brown smudge is left on the tub at the water
level. I had a filter put on my hot water heater (between the heater
and the tub) thinking that there must be impurities coming from the
water heater. Now I know there are none from the hot water heater -
the problem continues. I had a service tech from Kohler check this
out but he could not find the cause of the problem.