Is Your Goldfish Turning Black? 4 Reasons Why & What You Should Do

You see your goldfish turning black and you’re worried. But should you be?

What does it mean when your goldfish turns black? And what should you do about it?

Let’s find out in our big guide to goldfish turning black!

In this article, we reveal the top 4 reasons why goldfish turn black and – for each possibly reason – we explain what you need to do to keep your fish safe and healthy.

Why is my goldfish turning black?

It’s not unusual for goldfish-keepers to notice their goldfish turning black.

Black marks can appear on the head, the scales, the tail, or you may notice your goldfish’s fins turning black.

Cartoon of a goldfish with black patches all over
Goldfish turning black can be a big problem, so you should always work out why it’s happening.

Black marks suddenly appearing on your goldfish can be nothing to worry about, or they can be the sign of a serious problem. It’s therefore important to work out the reasons why your goldfish is turning black as quickly as possible and start treatment if necessary.

4 reasons why goldfish turn black

  1. Ammonia burns
  2. Healing from injury
  3. Natural coloration changes
  4. Black spot disease

Of these reasons, ammonia burn is by far the most common.

Unfortunately, ammonia burns are also extremely dangerous and often lethal. This is why it’s so important to act quickly to understand the reasons why your goldfish is turning black.

Below, we look at each of these four reasons in turn – including how to work out which reason is the one causing your goldfish to turn black and what you should do about it.

Goldfish turning black due to ammonia burns

Ammonia is a chemical found in your fish’s waste. It is highly hazardous to goldfish and can burn the fish all over its body.

When a goldfish suffers ammonia burn, its body tries to heal itself. This causes black patches on the goldfish in the same way that a burn on human skin might leave a scar.

An orange goldfish with black ammonia burn patches
A goldfish with ammonia burn

To an extent, seeing your goldfish turning black is therefore a good sign. It means that the ammonia burn is healing!

However, if your fish was burned by ammonia once, you need to take steps to make sure it doesn’t happen again.

How to tell if ammonia burn is the cause of your goldfish turning black

The first step to deciding whether ammonia burn is the reason behind your goldfish turning black is to do an ammonia test.

You should also test for nitrites and nitrates to see whether your tank is properly cycled.

High nitrite levels could be confirmation of ammonia burn, as they would suggest that your tank is not properly cycled and your fish may recently have experienced high ammonia levels. Nitrites themselves are also dangerous to goldfish.

We also recommend checking the size of your tank and confirming that it is suitable for the number and size of fish you are keeping.

If your tank is too small or over-stocked then you are likely to experience ammonia spikes, which will cause ammonia burn.

How to prevent ammonia burn

If your tank is set up well (including being properly cycled) and you perform regular water changes then there should be zero ammonia in your tank. Any ammonia released by your fish is quickly removed or turned into nitrites by bacteria.

However, if your tank is not cycled, contains too many fish, or is too small, then the ammonia levels will start to build up.

Unfortunately, it doesn’t take much ammonia to start burning your fish’s gills, scales and fins.

A goldfish living in water containing ammonia will become stressed, have difficulty breathing, and will most likely die quite quickly.

The way to prevent ammonia burn is therefore to have a big enough tank, stick to sensible numbers of fish, make sure your tank is properly cycled, and perform regular partial water changes.

Never add untreated tap water to your tank (use Seachem Prime first) and never wash your filter sponges in tap water. This can kill ‘good bacteria’ built-up during the cycling process and effectively un-cycle your tank.

Finally, test for ammonia, nitrites and nitrates regularly so that you always have a good understanding of your tank water conditions.

Goldfish turning black due to injuries

When a goldfish gets injured, it’s body attempts to heal itself, just like the human body does.

In humans, we see scabs and scar tissue developing at the point of the injury and we know this is a sign of healing.

In goldfish, this healing tissue is black. So, black patches on your goldfish can be a sign that your goldfish has been injured but is getting better.

If you know that your goldfish has recently been injured, and the black mark is in the same place as the injury, then this is almost definitely the reason why they are turning black.

If you don’t know of any injury, but think it’s possible that your goldfish could have been injured (for example, by a tank mate, in transport, or by an object in the tank) then you should watch your fish closely and remove the injury risk if possible.

If the injury is healing on its own – and that part of the goldfish turning black suggests that it is! – then there is usually no need to intervene and you can simply allow the injury to continue healing naturally.

Can goldfish be black or go black naturally?

It is possible for your goldfish to develop black spots, streaks or patches entirely naturally.

They may be born with black patterns or these patterns might develop later in life, along with other colors and patterns.

A goldfish’s coloration generally changes throughout its life – at least to some extent – and part of this change including black marks is not necessarily a cause for concern.

There are also some types of goldfish that are supposed to be black or partly black. These include the Black Moor and other types with calico patterns, such as the Shubunkin.

It is usually easy to tell when your goldfish turning black is simply part of a natural coloration change. In these cases, the change is usually gradual, the black marks don’t look sore, and there are no changes in the behavior of the goldfish.

However, if you notice your goldfish turning black suddenly, the black marks appear to be associated with an injury, there are red marks too or other physical problems such as frayed fins, or your fish starts acting differently, then this is unlikely to be natural.

When black marks appear suddenly or alongside other physical or behavioral changes, they are far more likely to be caused by ammonia burn or injury.

Black Spot Disease: Why does my goldfish have black spots?

As explained above, by far the most common reason for black spots, black smudges or black patches appearing on your goldfish is the healing of ammonia burns or injuries.

Black spots are not usually the sign of a disease, sickness or infection.

However, it is worth mentioning one possibility.

There is a disease known as ‘black spot disease’, which is a parasitic infection that causes black spots on fish.

But before you jump to the conclusion that your fish has black spot disease rather than ammonia burn, you should know that black spot disease is rare. (Unlike ammonia problems, which are very common!)

Your goldfish is only likely to suffer from black spot disease if it is kept in a tank with snails or in a pond. This is because the parasite’s lifecycle involves being passed in bird droppings, reaching water, where it infects snails, and then moving on to infect fish.

The infected fish are then eaten by other birds and the cycle continues.

As it is so rare, we strongly recommend considering other options before treating your goldfish for black spot disease. However, if you’re convinced that this is the reason for your goldfish turning black then a treatment such as PraziPro may help.

Featured image via Reddit.

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