As for fish, if it alters the water, I hope there is no harm in it.
Section: Ibn Aqil mentioned regarding one who strikes a permissible animal, it falls into water, and he then finds it dead, and he does not know whether it died from the wound or the water: the water remains on its original state of purity, and the animal remains on its original state of prohibition, unless the wound was fatal, in which case the animal is also permissible; because the apparent cause is its death by the wound, and the water is pure, unless blood fell into it.
Section: Animals are of two types: those that do not have flowing blood, and these are of two categories: that which originates from pure substances is pure, both alive and dead, and that is what we have mentioned. The second is that which originates from impurities, such as dung-beetles (15) and crickets; it is impure, both alive and dead, because it originates from impurity, so it is impure, like the offspring of a dog or a pig.
Ahmad said, in the narration of al-Marwudhi: Crickets of the privy and the cesspool, if they fall into a vessel or a large jar, it should be poured out; but the crickets of a well are not filthy, and they do not eat feces.
The second type is that which has flowing blood, and it is of three categories:
The first: that whose dead body is permissible, which is fish and the rest of the sea creatures that do not live except in the water. It is pure, alive and dead; if not for this, its eating would not be permitted. Even if it alters the water, it does not prohibit it, because it is impossible to avoid it.
The second category: that whose dead body is not permitted, other than the human being, such as land animals that are permissible to eat, and others, and sea animals (16) that live on land, such as the frog, the crocodile, and the like. All of these become impure upon death, and small amounts of water become impure if it dies in them, and large amounts of water if it alters them. This is the opinion of Ibn al-Mubarak (17), al-Shafi'i, and Abu Yusuf.
(15) The origin of "al-hash" is the garden (bustan), with the fat-ha (a) being more common than the damma (u), and calling a privy "al-hash" is a metaphor; for the Arabs used to relieve themselves in gardens, so when they constructed privies and used them as replacements, they applied that name to them. Al-Misbah al-Munir. (16) In M: "ka-haywan" (like an animal). (17) Abu Abd al-Rahman Abdullah ibn al-Mubarak al-Marwazi al-Hanzali, the Imam and ascetic, who gathered knowledge, jurisprudence, and literature. He died in the year 181 AH. Al-Jawahir al-Mudiyyah 2/324-326.
