Big Snapping Turtle, big tank....Does anyone have any suggestions on how to keep the water from smelling so bad? We've been cleaning the filter and completey changing charcoal and water every 3 days and still have an odor problem. It sort of smells like Sulphur, or a corpse! We have taken the water in to the petstore to be analyzed and bought the huge filter they recommended.....Any help will be appreciated.