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Why Is My Goldfish Turning Black? Causes & Solutions

Did you know that goldfish can turn black over time? It’s more common than you think but not every aquarist understands what this process means. Sometimes it’s entirely harmless – but sometimes it indicates a very real problem with your aquarium…

In this guide we talk about the most common reasons why you fish may be showing a dramatic color change and what you should do about it.

Why is My Goldfish Turning Black?

Here are a few of the most common reasons for goldfish turning black:

Natural Color Changes

The simplest and most likely reason is that your goldfish is simply maturing! They are notorious for shifting in color over time. Buying a fish that’s orange when young is no guarantee that it will remain the same color as an adult.

They may change to a black patchy pattern, grow in white scales, or turn entirely greenish brown or black. Typically, even black goldfish have a few orange scales remaining, though.

These changes are genetic in nature so it’s hard to know how your fish will look as an adult unless you buy a specific breed. And even if you buy a goldfish with good genes its color can still be affected by its diet, water conditions, and other factors.

So long as you are taking care to create an ideal habitat for your goldfish, don’t worry! Any new black scales are likely just the result of him or her getting older!

Ammonia Burn

Another common reason why goldfish turn black is due to ammonia burn. Unfortunately, this indicates a serious problem with your water conditions that needs immediate attention.

Ammonia is a waste product excreted by fish through their gills. It’s quite toxic to them but is broken down by your filter and environmental bacteria into nitrite. And this nitrite is broken down further into nitrate, the least poisonous nitrogenous waste product.

In a healthy ecosystem ammonia should be unmeasurable (0 ppm). You’ll not only have healthy bacteria but live plants that love to suck up ammonia as a ready source of nitrogen. Or else, you’re doing regular water changes to remove it.

Black scales often indicate ammonia levels so high they are causing literal chemical burns to your fish. This is especially common in goldfish bowls which have no filtration and therefore no place for beneficial bacteria to grow.

Goldfish bowls also allow feces and uneaten food to rot, causing ammonia levels to skyrocket.

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If you don’t already have one, you really should have an ammonia test kit on hand at all times. Ammonia is so toxic that even trace amounts affect the immune system and appetite of fish.

Even if a disease or other factors are causes for poor fish health, there’s a good chance ammonia may be making things worse.

How Do I Treat Ammonia Burn?

If your Goldfish has ammonia burn it’s displaying an advanced case of toxicity. Additional symptoms include reddened gills, inflamed red blood vessels showing, especially in the fins or eyes, and thick mucus patches on their sides.

Even if you see all of these symptoms you still have an excellent chance of saving your goldfish. You’ll need to do a massive (50%+) water change immediately and rethink your maintenance schedule.

Here are some common ways ammonia can become an issue:

  • Poor filtration: Do you have adequate filtration for your goldfish tank? Remember the beneficial bacteria that break down ammonia need space to live in. There are always some in the water column and gravel. But a good filter provides loads of surface area for additional bacteria and actively pumps water through it to feed them all that built-up ammonia!
  • Rotting Organic Matter: If you’re overfeeding, leftover food can find its way into crevices, where it rots. Bacteria can then release ammonia, which builds up alongside fish waste to cause possible ammonia burn. Do a thorough search for any source of rotting organic material to remove. This includes a hidden or buried dead fish, dead plant leaves, fish poop, and other organic debris.
  • Overstocking: Do you have too many fish? Or perhaps a few large fish in too small of a tank? Goldfish are thick-bodied fish and create more waste compared to a thinner fish that’s the same length. This is a perfect example of why the old One Inch Per Gallon rule doesn’t always work too well.
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Along with water changes ammonia absorbing resins are a great way to lock away excess waste products. These resin pouches fit right inside the filter unit, sucking up ammonia, nitrites, and nitrates until you remove them.

They are also rechargeable, ensuring they can be pulled out whenever your readings test a little high! Just remember that your nitrifying bacteria still need food too. So only pull these out during emergencies.


Once you’ve identified how ammonia is becoming a persistent issue and keep it down for a few months you’ll see the black scales start to fade. These kinds of injuries will heal and you’ll see the intense gold tones return!

Just remember not to get lax over water changes or filter maintenance! Goldfish are not only thick but very messy fish. They eat a lot, spit out crumbs, and root around on the bottom. So they need you to be extra-tidy for their own sakes!

Jason Roberts
About Jason Roberts
Jason is an aquarium fanatic that has been a fish hobbyist for almost three decades.

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