right to sort your ammonia out, partial water change and also clean your filter, its your filter that kills the ammonia, once u do that, go to pet shop and get something in the region of bacterlife(bacterlife is for fish tanks) u want the pond version. put that in as you will kill most of the bacteria in ur filter when you clean it.
if this fail u will have to take a fish out and examine for disease or parasites, there might b something on the skin, too much slime, flukes, or your fish might b in the early stages of white spot, would'nt worry about oxygen as the colder the water the more oxygen and we havn't hit summer yet, hope this helps