Two different subjects share the word. The water an animal lives in is a chemistry problem with published numbers behind it. The water an animal drinks is a health signal. They have almost nothing in common, so they are kept apart here.
Every aquarium test kit prints a number and almost nothing explains it. The same ammonia reading is close to harmless at one pH and lethal at another, and that is published, checkable, and almost never said. The pages here quote the document each figure comes from and link it, including the places where two respected organizations give different answers, because that is more useful than picking one and sounding certain.