I always use a little flash light and go over the snake very carfully as of doing this thuroughly i have caught mites still on my snake when it hought they was gone also or you could use a magnifying glass as they can be real small. I had a 8ft female redtail that loved her water bowl thats where shed always stay and shed get ticked you touch her in her when she was soakin shed hiss at you. Also keep a constant eye on the water while its clean for black dots, also have you check to see if your snake might be going into shedding mode ?