Fish are always letting out ammonia through their solid waste, urine, and gills. Uneaten food that starts to decay also causes ammonia. This may cause damaged gill membranes and prevent fish to have a healthy respiratory system. Very high levels of ammonia can cause fish to die. Even a little bit can cause stress, forcibly put an end to their immune system and causing fish loss and outbreaks of diseases. A level of lower than 1.0 is what you need to have a healthy tank.