Why is my Goldfish Turning Black? Is it Natural? Or Cause for Concern?

Many new fish owners worry they might not know if something is wrong with their much loved pets.

Unlike cats and dogs, they can’t signal to you that they may be injured or reacting badly to changes in their environment, but there’s no need to worry…if you pay attention to how your fish looks, you are very likely to be able to spot any warning signs fairly early on.

One such VERY obvious change you might observe can lead to the question: Why is my goldfish turning black?

Is it something to worry about? The sign of illness or disease? Let’s find out.

Why is my Goldfish Turning Black


If your Goldfish is turning black, try to pay attention to how much of their body is affected, and how quickly the change is taking place.

You should also think about your feeding and cleaning routines, and other environmental factors as this will help you narrow down what the underlying cause can be.

There may be a simple solution to the problem that will help your fish stay happy and healthy, or it may be a natural change that needs no action on your part.

Do I Need to Worry?

While some changes in color can be natural and no cause for concern, turning black can indicate trouble and you may need to make a few changes to see if you can help them.

As always, if you’re very concerned, especially if your pet is showing other symptoms along with the color change, you should seek specialist advice as soon as possible.

In most cases, there’s no need to panic if you notice your fish turning black. We’ve come up with a few of the most common reasons to help you work out if it’s a natural change, or an issue that needs your attention.

Changes to Suit the Environment

All fish have different cells in their skin, and some produce a black pigment called melanin. If you place your aquarium against a dark back drop, for example dark wallpaper, or if you decorate your tank with lots of dark items or rocks, then your goldfish may begin to produce more melanin.

If natural changes in color are the reason for turning black, you should notice the change on patches of the body, or in the fins. This type of color change is completely natural and normal, and is nothing to worry about.

It’s in the Genes

Many Goldfish owners will opt for a less expensive breed of the pet, and are happy to purchase a ‘mix breed’ to take home to their aquarium. If this is the case, you’re more likely to see color changes over time compared to if you’d bought a guaranteed color type from a reputable breeder.

While most of these color changes happen in the first year of life, you may still notice changes in coloring as your fish get older. This can include darker or black patches appearing, but also lighter areas showing themselves too.

If you have a ‘purebred’ goldfish in your aquarium, any color changes you notice are likely to be more subtle, and follow similar patterns to those of the parents. It’s still possible to notice changes in color though, especially while still young, so if you want to have a fish for its particular color, you may want to buy a slightly older one to increase the chances of that being the color they keep as they mature.

Ammonia Poisoning

High ammonia levels in your aquarium can lead to toxic poisoning and health problems, but the good news is, it’s fairly easy to fix this problem.

Ammonia is created as a by-product from the waste produced by your fish, but the levels they produce themselves is not harmful to their skin. The issues arise when excess food is also left in the tank to rot, as more ammonia is produced from this.

If high ammonia levels are the cause of turning black, this means the level has already been high and has caused small burns on the skin. As the ammonia levels drop, the damaged skin will start to heal, and this is when the black patches on the body will appear.

It is vital you don’t leave too much food in your aquarium – black patches on your fish mean that at least their skin has had time to heal from an ammonia burn. If the levels of ammonia are constantly too high, the skin won’t heal and the black marks won’t appear, so the damage may go unnoticed for a much longer period of time.

To help stop the initial risks of ammonia burns, remember to give your tank a thorough clean regularly, as removing some of the old water will help keep the ammonia levels low.

Black Spot Disease

This is a very rare problem, especially in adult fish, but it is still worth a mention. Black spot disease will only be the reason for turning black if you have water snails in the same tank, as they are the ones that carry it.

If you keep your goldfish in an outdoor pond rather than an aquarium, you’re more likely to encounter this disease, as they can catch it from bird droppings coming into contact with the pond.

Because this parasite affects the skin when their larvae try to burrow in, your fish will start to form hard cysts to protect themselves – these will look very dark or black. Another sign to look for if you feel this is the problem, is your goldfish may start to flick and rub themselves because of the irritation caused.

If you think the reason for turning black may be black spot disease, you need to remove the snails from the tank for a while to try and stop the parasite from continuing its life cycle.


The most common reasons behind a goldfish turning black are very natural and are no cause for concern, but it’s always worth considering the risk of disease and checking the ammonia levels in your goldfish tank.

Both of these issues can be solved fairly quickly and easily, and will make a huge difference to the life and health of your fish, so double check if they could be problems in your tank before putting any black spots down to natural changes in the pigments of the skin.

Happy fish keeping!

Wendy Kathryn

Hi, I'm Wendy, the owner and creator of this website, an experienced fish keeper and avid student of the art since 2010. My aim is to help beginners avoid the many possible mistakes when getting started in this wonderful hobby.

5 thoughts on “Why is my Goldfish Turning Black? Is it Natural? Or Cause for Concern?”

  1. Avatar
    Cheyenne Lynn

    I bought cheep goldfish from pet smart I have the proper care for them but they are turning black ALL over really fast. Is it normal I’ve had them for about a month now and ones almost completely black, ones halfway, and the others head is black. I’m scared because I’d really hate to be hurting them unknowingly.

    • Wendy Kathryn
      Wendy Kathryn

      Hi Cheyenne,

      If you have checked the water parameters, there are no high levels of ammonia, and your fish seem otherwise healthy in every way being active, feeding well, swimming actively, foraging, not lethargic, not gasping for air, etc., then it’s very likely a natural thing and nothing to worry about. I cannot gurantee this though without having ever seen and observed the fish, tested the water etc. but it sounds to me to be OK.

  2. Hi.
    I bought three goldfish from a pet shop in December 2018.
    They have all been very happy since and are all getting along fine.
    Over the past three to four weeks only one. The smallest is starting to get black fins and has a quite funny looking black top lip.
    I don’t think it’s the ammonia levels as the other two fish are showing no signs of turning black.
    Could it just be that this fish is meant to be black?

    • Wendy Kathryn
      Wendy Kathryn

      Hi Cj,

      Have you at least tested the ammonia levels?

      Anyway, it could be entirely natural and often is, but is very hard for me to say much of use ‘over the internet’ with no knowledge of set-up, water parameters, history etc. Just make sure to check all water parameters, keep an eye on his / her behavior and if nothing seems out of the ordinary, all behaviors are natural, eating well, swimming well, foraging, not lethargic or gasping for air anything…then (s)he is likely fine. If behavior changes at all or any other signs of illness surface, then you should investigate further.

  3. Avatar

    I changed my gold fish tank 3 weeks ago we use spring water(no tap water) when refilling the tank but know some of gold fish are turning black & the tank is cloudy I just bought a new double filter to see if that was the issue with the cloudy tank

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