"Halal" is a term in Islamic law that refers to any object or an action that it is allowed to use or take part in, so halal fish would refer to any fish that is permitted to be eaten. This law comes from the Islamic religious text, the Qur'an. Any type of fish with scales is considered to be halal and therefore permitted. Halal fish might come from either saltwater or freshwater.
There are several places in the Qur'an where seafood and fish are mentioned. In Al-Ma-ida, it talks of water-game, a reference to food found in the oceans, and An-Nahl refers to people who are associated with the sea being able to take produce from the water. Then in Fatir, both freshwater and seawater are mentioned, along with ruling that people are allowed to eat meat from both.
Halal fish must be caught when it still is alive. If the fish is found dead, it falls under the Islamic law prohibiting the consumption of animals that were found dead and is therefore prohibited. This refers only to the catching of the fish. Halal fish can be bought already cooked or processed fish, but adding certain ingredients or flavorings would make the whole thing forbidden. Anyone who is trying to purchase halal fish should check the ingredients of the fish against a list of halal ingredients to determine whether the fish can be eaten under Islamic law.
The Qur’an teaches that all animals should be treated with respect and should be well cared for. When the animal is killed, it should be subject to the minimum amount of pain and suffering. To be halal, the blood must be drained from the animal, because Muslims are prohibited from consuming animal blood. Any fish must also be subject to these rules for it to be considered halal fish. A practicing Muslim must follow this guidance strictly, and if there is any doubt about whether the fish is halal, it must be avoided.