I’ll put this right out there in the open:

MOST of the time if a goldfish is turning black, it’s not normal.

It’s almost always a sign of a serious problem that needs to be fixed right away, or you could lose your fish.

Today I’m going to explain why this happens and the steps you likely need to take to correct the situation.

Let’s get to it!

Why is My Goldfish Turning Black?

Contrary to popular belief, it’s NOT a disease when your goldfish turns black.

There are 2 possible things that cause a goldfish to turn black.

  1. Chemical burns from ammonia (most common)
  2. Natural color change

But the latter is very rarely the case.


When goldfish change color, usually black goes away.

It’s not common at all for them to go from, say, white or orange to black…

… but a black goldfish will often turn orange (because black is an unstable color in goldfish).

However, caustic burns from ammonia  in the water will injure the skin, fins, tail, gill plates, body and head of the fish and basically fry the area.

(Which is obviously not fun for the fish!)

This part you don’t usually see physically, but the fish’s behavior prior to the black markings showing up is often lethargic or irritated.

When you see it visibly turns black, that’s a sign of the fish trying to heal in that area.

Causes of Ammonia Burns


The #1 chemical that causes this problem for fish is something called ammonia.

It’s produced by the fish as they breathe and poop.

Ammonia is incredibly toxic to fish – so toxic that even the slightest trace can lead to burning of the fish.

But it gets worse…

It’s also VERY common in new tanks or bowls.

This is because any biological filter won’t be established for at least a month – meaning it won’t work to help clean the water for the fish.

Some people use carbon filters, which can definitely help…

.. But only to a point.

Overfeeding is a big problem with new fish, which can cause the filter to not be able to keep up with the amount of food (and consequently, waste produced by the fish) entering the water.

Another thing:

Stressed or sick fish will gas off tons of extra ammonia.

Unless a lot of water changes are done initially to help protect the fish from this, it can only continue to build up to massive levels in the water even with a filter.

The black can show up almost anywhere on the fish…

… But in many cases you might notice the black on the edges of the fins and the fins taking on a “pointed” appearance at the ends.

Other times you’ll see irregular smudges that look like patches appearing on the surface of the fish’s skin.

Related Post: Is Your Goldfish Turning White?

divider1- goldfish

What to Do to Cure Black Smudges on Goldfish

1. Test & Change the Water

The first thing I always do when I see black on a fish is to test the water.

You can get a simple ammonia test kit for this.

There are liquid tests and strips that you dip.

The liquid kind is most accurate, though the strips are easier to use.

I want to know specifically the exact initial reading of ammonia so I can monitor it daily going forward.

By the time you see black marks, in some cases the ammonia spike might have already settled – especially if you have a filter.

So don’t be surprised if you get a reading of 0 but still see your goldfish turning black.

Again, black is a sign of healing.

But many times the ammonia is present – or worse, off the charts.

If so, it’s time for an emergency water change, 50-100%.

Test again.

Usually it will be 0, but change the water as much as needed to get that ammonia reading to 0.

(Be sure to match the temp & condition the water).

Speaking of conditioning…

2. Dose with Prime

Next I dose the water with Prime to help bind the ammonia into a non-toxic form for the rest of the day.

In my experience Prime helps but it won’t fully protect the fish.

This brings me to the next point:

3. Add Carbon to the Water

Then I add carbon to the filter and keep that running or change out carbon if it’s older.

Carbon locks toxins in the water (including ammonia) to protect the fish.



4. Re-Test & Change

Each day, run another test for ammonia…

… And change the water if it’s anything above 0ppm.

Each 7 days I replace the carbon.


5. Limit Food

Reduce or stop feeding.

Food will cause more ammonia to be put into the water.

You can’t totally stop ammonia (as the fish constantly breathes it out) but reducing the food can help.

divider3 goldfish bowl

Will Your Fish Recover?

I’ve got good news and bad news:

The bad news is yes, if your goldfish is turning black, it probably has been burned by ammonia.

And if its behavior is not looking so hot right now…

… the prognosis for your fish’s survival isn’t that great.

This is because if your fish is also feeling bad, it’s most likely suffering from ammonia poisoning, which is usually fatal.

But there’s good news:

If your fish is STILL looking pretty perky despite being a bit fried, there’s a good chance it can make a full recovery.

Yes, the black will even go away with time and clean water.

Some fish are incredible fighters and some aren’t, a big part of it depends on the fish – and how much stress it’s already been through.

Many goldfish never get to the black marks stage – they are killed instantly by it.

So if yours got there, it’s already been very tough!

What About Black Spot Disease

Some people see black and immediately will tell you your fish has black spot disease.

Actually, that’s almost NEVER the case.

True black spot disease is very rare, and something you only really see in ponds for the most part.

It also has to be transmitted by snails.

The black spots also erupt to release a parasite (gross).

Literally, I’ve never come across this in person in all my years of fishkeeping.

So most likely you can cross this one off your list.

Final Thoughts

I hope this article has helped you to learn what it means when your goldfish turns black.

Now I want to hear from you.

Are you struggling with an ammonia problem in your tank or bowl?

Have you successfully reversed black smudges on your goldfish?

Let me know in the comments section below.