I'd like to add one more scenario for high ammonia concentrations, and that would be in watersheds or well fed ponds that have naturally high pH, in particular 9.0 or higher.
When the pH is high that means there are huge amounts of free hydroxyl groups (OH-) that will rob the hydrogen ion from ammonium (NH4+) yielding unionized ammonia (NH3) which is the form that is toxic to fish.
Another factor leading to high pH and consequently more unionized ammonia is the diurnal cycle of plant respiration and photosynthesis. Photosynthesis is the process whereby organisms convert carbon dioxide and water into carbohydrate and oxygen. The net reaction is:
So there is net consumption of carbon dioxide during the day. This can lead to some ponds becoming deficient in CO2 during the day, raising their pH.
Incidentally, this situation can be remedied by adding alum. When aluminum sulfate reacts with water there are three H+ ions liberated which have the ability to turn unionized ammonia back to it's safer, ionized form.
Holding a redear sunfish is like running with scissors.
