When I have back to back stone sessions I spray the stones and the water with a "spa oxidizer" consisting of 1 part chlorine bleach, 1 part white vinegar, and 1 part water and then turn the stone warmer on high for a bit to disperse the solution. ย This is the same solution used at some spa's to sanitize hot tubs. ย At the days end the stones can be scrubbed with germicidal soap, and water using those exfoliation gloves which is quicker than using a scrubbie.