If you take a strong mineral bath... and have not for a while, the water will show color after a time.
for me, Usually baking soda and/or potassium bicarbonate, magnesium sulfate and H2O2... a dingy non see through cloudy grey or smoggy brown is not uncommon.
In addition, you may start to see oils as you do with the foot bath.
This can take much longer than 30 minutes though and most people do not stay in for more than the recommended 20 minutes.
magnetic rings among other things can help to eliminate these things too... by breaking up ionic bonds.