All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The water you referred to in the question is pure because the fish is pure and whatever comes out of it is also pure as it comes out from an animal whose meat is permissible to eat.
The most preponderant opinion in our view in Islamweb is that the urine and dung of an animal whose meat is permissible to eat are pure as we clarified in Fataawa 91362 and 91392.
Moreover, if the fish dies in water, the water does not become impure because the dead fish is pure as mentioned in the Hadeeth: “There are two dead animals that are lawful for us (to eat): they are fish and locust.” [Ahmad and Ibn Maajah] For more benefit, please refer to Fatwa 131587.
Allaah Knows best.