Some groundwater predates animals, some volcanic steam, and water from recent comets would be less likely to have passed through an animal, but in general, most water on Earth has passed through several fish at the very least. The mass of the organisms is not what matters...rather how much they take in over time.
Just one human produces 82,150kg of crap in a lifetime. We eat fairly nutritious stuff. Many animals have to eat stuff that is much less nutrient dense and produce far more for the mass equivalent.
Not to worry, H2O is H2O...as long as it has the right number of protons, neutrons, and electrons. It could have gone through a billion anuses and it would be no different.