وَمَوْتُ مَا يَعِيشُ فِي الْمَاءِ إِذَا مَاتَ فِي الْمَاءِ لَا يُفْسِدُ الْمَاءَ، كَالسَّمَكِ وَالضَّفْدَعِ وَالسَّرَطَانِ.
Meaning: If an animal that lives in water dies in water, it does not spoil the water, such as fish, frogs and crabs.
Animals that normally live in water form a separate juristic category. Under the Hanafi rule cited here, their death in water does not by itself spoil that water.
Fish, frogs and crabs are examples; the ruling differs from simply applying the case of a land animal falling into water.