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. This is the same solution used at some spa's to sanitize hot tubs. 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.