Re: Fish Urea Output and Ammonia Solution Concentration Levels, Aquaria

I have to admit that I didn't know where to begin with this one, so I asked 
the resident expert in our laboratory, Bruce Woodin.  This man not only 
knows his fish physiology, but also worked in an aquarium shop while in 
school.  This was his answer:

Teleosts primarily excrete ammonia (not urea), which is toxic, but
generally not a problem after dilution in an open system.  In a closed
system (aquarium), bacteria utilize the ammonia to synthesize amino
acids and thus eliminate it from the aqurium water in an established
tank.  The reversible reactions involved require 2 protons, which become
limiting in an alkaline environment (favoring ammonia remaining as
such).