What form of nitrogenous waste do most fishes excrete?

Most fishes primarily excrete ammonia as their nitrogenous waste. Ammonia is highly toxic but soluble in water, allowing it to be efficiently eliminated through the gills. This method is effective for aquatic animals since they have a constant supply of water to dilute the ammonia. Some species, especially those in saltwater environments, may also convert ammonia into less toxic compounds, such as urea, but ammonia remains the predominant form.