Visible impurity is material whose presence can be seen; invisible impurity is not identifiable as a distinct visible object after entering water.
Invisible does not mean automatically harmless. Jurists consider whether impurity remains, what effect it produces and from which point the water is taken.
Those questions account for different interpretations within Hanafi fiqh regarding the point of entry in running water.
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Impurity in Stagnant and Running Water, Large Ponds, and Aquatic Creatures
Visible impurity is material whose presence can be seen; invisible impurity is not identifiable as a distinct visible object after entering water.
Invisible does not mean automatically harmless. Jurists consider whether impurity remains, what effect it produces and from which point the water is taken.
Those questions account for different interpretations within Hanafi fiqh regarding the point of entry in running water.