What is the ruling on performing ablution from water in an underground tank in which a snake has died and from which a faint odor has emerged, and is this water considered impure or pure?

A dead snake is impure (najis). If the water in a tank changes due to it, even slightly, it is considered defiled (mutanajis), and purification with it is not valid. This is because a large quantity of water becomes impure if its properties (color, smell, taste) change due to contact with an impurity, whether the change is slight or significant.