Why Do Goldfish Turn Black

Why Do Goldfish Turn Black? | 5 Common Triggers

Why do goldfish turn black? This is a question you are likely to ask yourself if you are not well acquainted with the care of your pet goldfish.

Goldfish is one of the most popular pets for an aquarium as they look the most attractive while inside an aquarium.

There are a lot of other benefits of including goldfish in your fish tank. But if you are to include goldfish, you should learn to take optimum care of them so that they remain happy and healthy.

Nowadays, more and more people are keeping an aquarium in their homes. Aquariums are great for improving the aesthetics of the house, relaxation, and are also a great conversation starter when you have visitors. Having goldfish in your tank makes it attractive.

Obviously, any time a fish turns black, this can be that your Goldfish is not in good health and your immediate attention is required to investigate what might be the cause.

But what are the factors that contribute to this? Why do goldfish turn black?

Before we answer that question, let’s first justify our urge to grow goldfish.

Why do gold fish turn black

Benefits of Having Goldfish As Pets

The following are the benefits of having goldfish in your fish tank:

  1. They are among those fish species which are easily available for people to keep as pets. Some other species of fish may not be readily available locally in the aquarium stores.
  2. They also fall under the class of fish considered as low maintenance.
  3. They also aren’t expensive and do not have a specialized diet and their ideal food is available in most supermarkets and aquarium stores.
  4. Keeping goldfish in your fish tank is also a hobby which you can easily commercialize by breeding them and then offering them for sale.

Why do Goldfish Turn Black?

Goldfish are the most colourful species among all the aquarium fishes. Including them in your fish tank gives a beautiful contrast of colors which is great to look at.

In the due course of time, the nature of the goldfish can change or vary because of the aquarium conditions. Why do goldfish turn black?

These are 5 triggers which are fairly common in triggering their change in color.

1) Ammonia Burns up

This is actually the most frequent reason that they change color. Ammonia is produced inside the fish tank due to excess fish food in the tank which eventually decomposes if the water in the tank is not adequately filtered/Cycled.

The ammonia itself does not cause the skin to turn black, but the healing process which happens naturally turns their color to black.

In the event the water isn’t correctly filtered and cleaned out frequently, the toxins may cause burns up to the goldfish. That black color is actually a sign the infection is healing, and so cleaning out the container correctly can help make sure the infection does not come back.

With time, that goldfish should certainly go back to normal color.

See also: What can fish eat besides fish food

2) Change of color caused by environmental factors

The location of your aquarium can also affect the color of the skin of your goldfish. It is advisable that an aquarium should be kept in a well-lit area.

This is because when the fish are kept in a dark area they tend to secret a black pigment known as melanin which may appear as black patches on the skin of the fish

3) Temperature of the water

It is important to select the location for your tank away from sources of heat such as direct sunlight and high bulbs and lamps that may emit huge amounts of heat when in operation.

Small tanks will also tend to heat up more quickly when exposed to hot environments. That said, this is not to overlook the importance of a heater in your goldfish tank.

4) That tank is actually small

The fish tank might be too small which may lead to a faster saturation of ammonia and carbon in the tank due to the low water volume present. It is common to start out with small tanks as most of the fish bought from Pet stores are usually small in size.

The downside is that as the fish grow in size we tend to overlook the need to move them to an appropriately larger size tank.

Buying a 10 or 20 gallon fish tank container for your fish is usually adequate to ensure the goldfish has lots of space to thrive in the water.

The ideal tank size will be dependent on the quantity and size of fish you put in the tank. For larger sizes and quantities, you could even scale up to 40 Gallon breeder tank.

This rapid build-up of toxins in the small tank present greatly increases the chances of your fish contracting disease.

5) Disease May cause Goldfish to Turn Black

Including water snails along with the goldfish in the same tank is a bad idea as the snails considerably increase the chances of your fish contracting the disease from the snails.

There is a disease known as schistosomiasis which is a parasite carried by freshwater snails and is spread by water contamination. The eggs of the parasite causing this disease, burrow deep into the pores and skin of the Goldfish and form protective hard cysts that are black in color.

The consequence of this infestation causes the overall look of black spots in the Goldfish.

Why Do Goldfish Turn Black

Optimum Care and some precautions to avoid Problems

After finding out why goldfish turn black, you need to find the optimum solution. 

But you don’t need to panic every time your goldfish develop some dark patches on their skin as the patches often tend to clear up quickly sometimes there is not much going on with the fish.

It is also important that sometimes goldfish will naturally change color for short durations due to reasons that are purely natural for them.

If you can’t quite pinpoint the exact cause of the change in color, it is advisable to consult with your veterinarian for professional advice.


Indiana Department of Natural Resources: Fish Disease- https://www.in.gov/dnr/fish-and-wildlife/wildlife-resources/wildlife-diseases-in-indiana/fish-disease/

Hannah Nichols; University of Idaho: Goldfish- https://www.lib.uidaho.edu/digital/objects/guidedreading/guidedread033.pdf

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