My uncle has recently finished moving into a house that came with a 400 gallon pond with 9 koi and 6 or so presumed goldfish (no barbels, plus they're smaller, but have a similar body shape). One of the goldfish is looking very lethargic, with torn caudal fin(s), spots on its body that look like red burns, (the fish itself all white save for a red splotch on the head), inflamed gills on one side and a fluffy coating on that same side. It just looks really pitiful and my aunt and I really want to save it (or put it out of its misery, as the case may be). She tested the water and came up with safe parameters except for ammonia and nitrate, which were both a bit high. Help!