system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are pet fish okay living in perfectly clean, filtered water, when they normally come from natural bodies of water that are not so pure?
gpt
Firstly, because it's not just about cleanliness it's also about oxygen content. Natural bodies of water will absorb oxygen from the atmosphere, but a small fish tank, without some kind of filtration system, will end up depleted of oxygen.

Secondly, it's a specific kind of "impurity" in your fish tank that hurts the fish. Their water is filling up with their own feces. If any natural body of water were as full of feces as an unfiltered fish tank, those fish would be sick and dying too. Other kinds of impurities are more tolerable for the fish, not harming their health even though they wouldn't be good water for you as a human to drink.

Also though, natural bodies of water that become heavily polluted will eventually see the fish and other wildlife in them die. Even parts of the ocean can become so polluted that they are "dead zones" where few if any organisms live.