Pond goldfish possible disease

Question
I have a canary yellow goldfish in our pond, along with two other canary goldfish and 3 shubunkins.  One of the goldfish has developed black spots all along it's sides, and black smudges at the gills.  None of the other fish seem affected at all, and the fish in question is not exhibiting any unusual behaviour.  Any ideas what it is [injury, perhaps?] and do I need to do anything about it?

Answer
Hi Pat;

I don't usually answer 'pond' questions, but goldfish are goldfish so I will give it a shot.

The black is either a natural color change, or is a response to burns from previously elevated ammonia and/or nitrites (toxins). Is the pond fairly new, recently cleaned or the filter recently cleaned? Were the fish recently added? These things will cause spikes of toxins.

When fish are burned by toxins, it shows up as black 'scabs' and will heal in time as the damaged tissue is replaced with new tissue. Not all fish exposed to toxins will show the black either, so it can be a confusing symptom. Once you see the black, the toxins are usually gone already. This means that you won't always see a positive result on an ammonia or nitrite test kit if you don't know about it before the black scabbing occurs.

Since the fish seem to be acting okay, I wouldn't worry. Just to be safe, it might be a good idea to have the water tested though. The local fish store can do that for you.

Followups welcome

At Your Service;
Chris Robbins

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