Humans excrete urea in their urine and fish excrete urea also including through the gills. I don't know if fish excrete all their urine through the gills but it is this that makes men sit cross legged when thinking about a little fish that tracks the urea and ends up in the wrong place with humans if they swim in its habitat. Bacteria change nitrogen compounds like urea to ammonia, nitrite, nitrate and back again. I am not sure if fish excrete ammonia also.