Typical symptoms of goldfish and koi diseases. ... BLACK SPOTS The most common cause of black spotting on fish is transient ammonia spikes.

Black spots in goldfish, koi , and other coldwater fish often occur as a result of ammonia surges in the water. The black spots can appear on the side or back of your fish. Frequent partial water changes will help to correct this condition and the color on the goldfish should return to normal within a few weeks.

I raise KOI so, take it from one who knows.