I say, with God's guidance: If impurity falls into a cistern or water tank, it should be considered: If its water is less than ten by ten — which equals (25) square meters of water surface area, and a depth where the ground is not visible when scooping — all the water must be poured out. However, if the water surface area is ten by ten or more, its ruling is like that of flowing water or a large amount of stagnant water; it does not become impure unless the impurity affects its taste, color, or smell, even at the point of occurrence. If the impurity has a physical form, being visible and apparent, one should not perform ablution from that spot. And God knows best.