Too much water can result in loose stools, as can other things. As long as the rosy is digesting and not regurgitating, then I would not worry about parasites. Treat the mites and keep the cage dry. Under tank heat, a thermostat and a substrate of aspen or paper works well. If you have a screen top then you can have a water dish in there all the time. But if using a tub, only have water in there when the snake has not fed recently.