Effect ok - but leaves the appliance discoloration

The foot bath crystals can be metered easily. The effect is ok and it has a pleasant smell. Would actually awarded five stars, but the fact that discoloration caused by the crystals to the device materials, talking about it. In my new foot bath, in which the surfaces are still very smooth, these deposits can be removed relatively easily. If not so smooth surfaces but I can not imagine that the material does not discolor me.