The Hanafi rationale is that creatures lacking flowing blood do not produce the same blood-related impurity associated with the death of larger land animals.
Their rule is therefore not simply analogised to every other death in water.
A separate source of impurity in the water must still be assessed under its own rules.
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Impurity in Stagnant and Running Water, Large Ponds, and Aquatic Creatures
The Hanafi rationale is that creatures lacking flowing blood do not produce the same blood-related impurity associated with the death of larger land animals.
Their rule is therefore not simply analogised to every other death in water.
A separate source of impurity in the water must still be assessed under its own rules.