What is the ruling on feeding dead catfish to birds and selling them to hotels, and is it permissible to purify them by feeding them plant-based feed before selling them?

Scholars have differed on the ruling of feeding edible animals impure substances. Some, like the Shafi'is, consider it disliked (makrooh). Others have elaborated, stating that it is permissible to feed impure substances to an animal whose meat is eaten if it is not going to be slaughtered soon. However, it is not permissible if it is going to be slaughtered soon, for fear that it might become a jallalah (an animal fed on impurities) and its consumption would be prohibited. This is the preponderant opinion in the Hanbali school.

Accordingly, there is no harm in using dead chickens to feed fish, provided that feeding them with it is stopped a sufficient time before selling them, so that the effect of impurity disappears and their meat becomes wholesome. This is because a jallalah animal, once the effect of impurity is gone, becomes permissible to eat by consensus.