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Goldfish Turning Black: 3 Reasons Why It Happens

Renee Whitmore
Last Updated on
by Renee Whitmore

A goldfish that is turning black

Taking a second glance over at your fish tank only to see your goldfish sporting black spots can seem alarming to any fish parent. But there’s no reason to get overly concerned just yet.

There are a handful of reasons why your goldfish appears to be turning black, some of which are easily manageable.

Color block changes in goldfish are a rare occurrence, but you’ll need to investigate all the possible causes to know the best course of action. There’s no need to assume a darkened goldfish is a severe health problem before assessing further.

Why Is My Goldfish Turning Black?

Your golden buddy suddenly turning black can be scary and concerning, but how concerned you should be depends a lot on the reason behind this transformation.

There are three main reasons a goldfish might turn black:

  • Ammonia
  • Genetics
  • Disease

If ammonia or disease is the root cause of your goldfish’s color change, you will need to take steps to reverse the issue and help your fish. However, if their genetics is the reason behind the change, then your goldfish may no longer be gold, but they will be fine!

Let’s take a closer look at how to identify and fix the three possible causes.

Ammonia in the Tank

High ammonia levels in your fish tank are among the most common issues contributing to a goldfish turning black.

If this is the case, the discoloration will begin with the goldfish’s fins and spread to the rest of the body over time. You may notice some small patches of black skin that look like burns.

Ammonia buildup in a fish tank is a common problem caused by fish waste, decaying plant matter, and uneaten food left in the water. This buildup can be prevented by regularly cleaning your aquarium and installing a filtration system.

Ammonia can be highly toxic to fish, even in small amounts, so it’s crucial to keep your fish tank clean regularly. The ammonia can quite literally burn through your goldfish’s skin, and if the levels get even higher, the fish will eventually die as the gills get burnt off entirely.

However, the discoloration you see can be a good sign and indicates that your goldfish is healing from the burns. To be sure, you’ll want to check the ammonia levels in the water using a test kit. Anything above 0 PPM should be a cause for concern.

It’s also important to note that ammonia-induced discoloration isn’t limited to goldfish and can happen to many different light-colored fish species. 

Be sure to change the water, remove any dead plants or excess food, and only feed about as much food that your goldfish can eat within a couple of minutes to prevent another ammonia spike.


If you’ve checked your tank’s ammonia levels and cleaned it out only to still see no improvement, another potential cause of your fish’s discoloration may be its genetics. Many specimens of goldfish are genetically prone to color block changes.

These genetics can appear as a result of mixed-breed goldfish that are typically priced lower at pet stores and the like. Some of these fish develop dark spots during their juvenile stage, while others may show them later in their life cycle.

These dark spots can accompany other color changes like different hues of orange and yellow.

Still, pure-bred goldfish from a reputable seller may develop some discoloration, although it would appear much more faintly.

So, if you believe genetics is the cause of your goldfish’s discoloration, you can rest easy knowing it’s nothing to worry about. Their genes are unique to each fish and don’t usually cause any health defects or shortened lifespans.


The third common reason for your goldfish turning black is a rare fish disease called black spot disease. Typically, this ailment occurs in wild goldfish found in outdoor ponds and encountered by fishermen more than inside home aquariums.

Black spot disease is a parasitic fluke disease that fish often contract from water snails. Goldfish may also catch this disease from bird droppings contaminating the pond water.

The discoloration from black spot disease will look like just that: black spots. The number of spots will vary from fish to fish, depending on the severity of the disease. A more serious infection could significantly cover the goldfish in dark spots.

The spots in question aren’t discoloration on the fish’s skin but rather parasite eggs burrowed inside the skin. The eggs then develop into hard cysts and can eventually burst while inside the fish’s skin.

Your goldfish’s behavior can help determine whether or not the discoloration is black spot disease or something else. The spots tend to feel uncomfortable and itchy, causing the fish to flick their bodies or rub against items in an attempt to alleviate the discomfort.

Treating this disease involved removing any snails you might see in the fish tank or pond. Over time, the fish will recover as the parasites’ life cycle ends.

Like we said, this disease is super rare if you’re keeping your goldfish in an aquarium rather than in an outdoor pond, so you don’t need to worry about it too much.


If you find yourself thinking, “why is my goldfish turning back?” there’s no need to panic! Carefully assess the situation, the behavior of the goldfish, and always clean out your fish tank.

You should also always check your tank’s ammonia levels regularly and ensure your filtration system is working correctly.

It will depend on the cause of the discoloration to determine whether or not a goldfish will return to its original color. For example, if the reason is genetic, it likely will not turn back, but this will not affect its health.

If the cause is due to ammonia poisoning, the fish could heal. As we said, discoloration is actually a sign of this.

However, if the ammonia levels are too high, the main concern here is whether or not the goldfish will survive. If your goldfish’s swimming behavior has changed along with the dark patches, it may not be on the way to recovery.

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About Renee Whitmore
Renee Whitmore
Renee Whitmore is an American college professor and freelance writer from North Carolina. She holds a Bachelor’s Degree in English and a Master’s Degree in English Education. When she is not driving her teenage son to wrestling practice or learning the ins and outs of Fortnite from her younger son, she is working on her first book to be published soon.
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