Do you feed crickets? When I used to feed crickets I would occasionally see dozens of tiny, tiny baby crickets in the water portion of my pyxie's tank. If any crickets go uneaten and lay eggs, or eggs get thrown in the tank with the crickets, they can develop seemingly over night. Plus, most mites that infect reptiles are really small, close to microscopic. I don't know that you would see them if they got into the water.