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Why Is My Goldfish Turning Black? 3 Common Reasons

Lindsey Stanton Profile Picture

By Lindsey Stanton

gold fish turning black

Goldfish are some pretty neat creatures no doubt, and there are actually quite a few different kinds of them. Generally speaking, they tend to be fairly easy to take care of, but there are some things that you must watch out for. Yes, these fish are named goldfish, but they can change colors. Sometimes they change color naturally, and sometimes it happens due to environmental causes, such as aquarium conditions. fish divider

The 3 Reasons Why Your Goldfish is Turning Black

Let’s talk about why your goldfish fins are turning black. Plus, whether it makes a difference, and if necessary, what you can do about it.

1. A Natural Fin Color Change

First off, goldfish will change color depending on their location and surroundings, especially when lighting is concerned. Goldfish have different types of cells located within their scales that produce different colors. Some of these cells produce melanin, the primary compound which causes dark skin colors in people.

What is interesting to note is that goldfish that live in fairly dark environments with dull surroundings and little or no lighting often change to a blackish color. Some goldfish only suffer from their fins changing black, while there are more extreme cases where the whole fish turns black.

Now, this is not a health issue per se and it really won’t affect your goldfish. Heck, your goldfish probably won’t even notice. However, although it is not a health detriment, you might mind the color change. The simple solution here is to add some brightness to your goldfish tank. Increase the lighting and add some colorful decorations. This should take care of black fins, and after several weeks, they should return to their original color.


Butterfly Goldfish
Image Credit: stupphips, Shutterstock

2. Ammonia Burns

Another well-known cause of goldfish fins turning black is ammonia burn. Ammonia is lethal to fish in even small quantities. It poisons them, suffocates them, and will kill them quite quickly if not taken care of. Black fins on your goldfish are an early warning sign that the ammonia levels in your aquarium are way too high. In fact, any ammonia is too much.

If your goldfish fins are turning black, you should test for ammonia right away. What is interesting here is that the black color actually signifies that the ammonia has caused a skin infection, one that is actually starting to heal. It may sound weird that black fins are a sign of healing, but it can go either way. They might heal, or the high levels of ammonia might continue to wreak havoc on your fish.

Either way, you need to make sure to take care of the ammonia problem.

What Causes High Ammonia Levels?

High levels of ammonia are caused by uneaten food that is decaying, decaying plant matter, and fish waste. In other words, high levels of ammonia in any fish tank are more or less a result of improper tank care. You need to clean your tank regularly and remove all debris. At the same time, having a biological filter that works well is a big bonus too.

Biological filtration units take care of ammonia with surprising efficiency (more on lowering ammonia levels over at this article). On this same note, tanks that are too small, aren’t cleaned well, and have a high level of toxins can also cause goldfish discoloration, even those black fins we are talking about. The best solution is to get a larger tank, get a good multi-stage filter, and make sure that you always clean the tank.

Besides discoloration, a dirty and not well-maintained tank can have other serious health effects as well.

3. Disease

Although it is rare for goldfish fins to turn black due to disease, it is possible. There is a type of parasite that can infest fish tanks, a parasite that usually hitches a ride with snails.

So, if you have a goldfish with fins that are turning black, and you have snails in the tank, chances are that this parasite is the cause. This parasite lays burrows under the skin of your fish, it then lays eggs, followed by the creation of a black and hard cyst, thus the black fins. If you want to get rid of the black fins, the parasites, and have things go back to normal, removing the snails from the tank should do the trick.

It might take a month or two for this solution to take effect, but it will work. Also, if the problem appears very severe, using some kind of aquatic parasite treatment will work as well. It will take care of the problem faster, but it will cost some money.

ryukin goldfish
Image Credit: Galina Savina, Shutterstock

If you suspect your fish is sick and want to ensure you provide the right treatment, we recommend that you check out our best-selling and comprehensive book The Truth About Goldfish on Amazon today.

The Truth About Goldfish New Edition

It has entire chapters dedicated to in-depth diagnoses, treatment options, a treatment index, and a list of everything in our fishkeeping medicine cabinet, natural and commercial (and more!)

NOTE: If your goldfish is swimming upside down, then this post will explain why.

Commonly Asked Questions

Is it normal for goldfish to change color?

Yes, it is perfectly normal for some goldfish to change color. Goldfish do naturally change color as they age, but this does usually happen within the first 18 to 24 months of their life. Some black goldfish may start to change to a yellow or orange color as they age, and some lose their black markings on their fins and bodies, which is perfectly normal.

However, what is usually not normal is if your goldfish starts changing from yellow or orange to black, or other such colors.

goldfish fish aquarium
Image Credit: Ammit Jack, Shutterstock

Why is my goldfish’s tail turning black?

If your goldfish is turning black, especially its tail, you might have a problem. Those black spots may very well be ammonia burns due to ammonia being in the water. Realistically speaking, for healthy fish, there should be absolutely no ammonia in fish tank water. This ammonia can relatively easily burn your fish, and this goes for all fish, it’s just that for goldfish, those black spots are really noticeable on their yellow or golden coats.

Some people will say that up to 2 parts per million of ammonia in a tank is acceptable, but the only really safe level of ammonia in a fish tank is 0. If you see black spots developing, on the tail or elsewhere, you need an ammonia test kit and you need to rectify the situation immediately, or else the goldfish will die, and quite quickly at that.

Will black spots on goldfish go away?

If you manage to get rid of the ammonia in the water, get rid of diseases, pests, and whatever else, and the injury heals, then yes, those black spots will go away. However, if you don’t fix the problem, such as high levels of ammonia in the water, then not only will those black spots not go away, but they will also get worse, and eventually, your fish will die.

Why is my goldfish’s mouth turning black?

This is the same problem as the goldfish’s tail turning black. Black spots anywhere on your goldfish are a result of ammonia in the water. You absolutely need to take care of high ammonia levels in the water if you want those black spots around the mouth to go away and you want your goldfish to survive.

Goldfish turning black on fins and body

If your goldfish is turning black on the fins and body, the tail, the mouth, or anywhere else, you need to check for ammonia and you need to control it right away, or else it will be fatal.

goldfish in tank
Image credit: Unsplash

Why do goldfish lose color?

Goldfish will naturally change their color over time, and as they age, they often lose a lot of their black coloration and may turn a greyish color. They will usually change color in the first year or two of life, and this often involves losing some of that color. Now, this is not the only reason. Goldfish scales are known to become much duller and less colorful when they are deprived of light. Moreover, losing color may also have to do with an insufficient diet.

How do I increase the color of my goldfish?

You can increase the color of your goldfish by providing them with color-enhancing foods. There are various foods out there that are very rich in many vitamins and have essential oils and fatty acids in them which can go a very long way in increasing the color of your goldfish. Moreover, something that goes a long way in increasing the color of goldfish is to provide more lighting. Now, more light may not increase the color of goldfish per se, but it will stop the color from fading as much.

Something else that will help is simply if your goldfish is healthy, which means providing them with proper water parameters and very clean water.

Related: 5 Reasons Why Your Goldfish is Turning White. wave tropical divider


Black goldfish fins can be caused by various things, the most common of which is low lighting. However, ammonia and other toxins are always problems, to make sure to test for them and to keep your tank clean and well maintained. If you have snails, it’s probably a parasite causing the color change, so take care of that if needed.

Related Read: Why Is My Goldfish Turning Red?

Feature Image Credit: HamsterMan, Shutterstock

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