In most freshwater fish, nitrogenous waste is primarily excreted as ammonia, which is highly soluble in water and is toxic at low concentrations. In terrestrial mammals, ammonia is converted to urea before it is excreted. Urea is also highly soluble in water but is less toxic than ammonia at low concentrations. Which of the following best explains why freshwater fish do not convert ammonia to urea for excretion?

Terrestrial organisms typically need to conserve water. Nitrogenous waste is converted in order to reduce water loss. Freshwater fish do not need to conserve water. In typical habitats, the amount of water in the environment is more than enough to dilute the toxic ammonia.

If you were to hold a goldfish in a small bowl, the ammonia waste would quickly lead to the death of the fish.