Why Is My Goldfish Turning Black


Why Is My Goldfish Turning Black?

Since goldfish are considered low-maintenance pets, I was shocked when I glanced at my aquarium one day and noticed my goldfish turning black!

I immediately began looking for an answer to the question running frantically through my head: “Why is my goldfish turning black?”

Apparently, there are several reasons goldfish develop black spots. Most are either harmless or easily correctable, though these spots can occasionally be dangerous.

Let’s look at the various causes and determine how to diagnose and treat black spots in your fish so it can live a long and healthy life:

Why Do Goldfish Turn Black?

Blending into the Environment

One reason your goldfish might be turning black is due to its environment. Goldfish will occasionally start changing colors, which can include producing more melanin or dark coloring, to match their surroundings.

Your fish may develop black markings if it’s surrounded by a lot of dark rocks, foliage, or other tank decorations, or if the aquarium is near a dark-colored wall.

Mixed Breeding

Another reason goldfish can turn black is due to their genetics.

The name “goldfish” is mildly misleading, as these fish can also come in red, yellow, brown, orange, gray, white, and even black. Many fish, especially those who have not been bred specifically for pure gold color, often have a mix of colors that can change over time.

Pure goldfish is the most expensive breed, and many aquatic enthusiasts choose to buy a less expensive, but therefor mixed, breed.

Some owners may not realize they have a mixed breed at first. Goldfish tend to develop their colors throughout the first year of their lives. However, goldfish can occasionally develop a shift in colors or have new color spots appear even in adulthood.

So, if you find yourself wondering “why is my goldfish turning black?” the answer may be that it’s in its genetics, and its mixed breeding is starting to show through.

Chemical Burns from Ammonia

Unfortunately, the answer to “why do goldfish turn black?” is sometimes that they are suffering from ammonia chemical burns.

Ammonia is caused by food breaking down in your fish’s aquarium, and it is also the natural by-product of your fish filtering oxygen through its gills and creating waste in the water.

Low levels of ammonia are harmless, but if the levels get too high – which often happens if too much food is left in the tank or the water isn’t changed enough – it can cause chemicals burns.

The black spots actually aren’t the ammonia burns themselves but are instead a sign that your fish is starting to heal. This doesn’t mean that the danger is over, though. You may still need to fix the ammonia levels in the tank and monitor your fish closely to make sure it’s healing properly.

Black Spot Disease

Another potentially worrying answer to the question “why do goldfish turn black?” is occasionally that they have black spot disease.

Often, if people notice goldfish turning black, they immediately think of black spot disease. The truth is, though, that this disease is pretty rare for goldfish, especially if they are kept in an aquarium. It can be deadly, though, so it’s good to rule this out as a possibility.

Black spot disease is caused by parasites carried by water snails. The larvae of these parasites like to try to burrow into the skin of goldfish. In response, the fish will form a hard, black cyst around the larvae to try to protect itself. These cysts can look like random black spots.

If you keep your goldfish in an outdoor pond, they are more likely to contract the disease as the parasites can be transmitted through bird droppings that may fall into the water.

Diagnosing the Problem

Natural Causes

If your fish’s dark markings are symmetrical and near its extremities, these are probably naturally-occurring color changes.

If your fish has a lot of dark colors in its environment, try removing these things or replacing them with more brightly colored options and monitor you fish for a few weeks. The spots might start clearing up, or they may just be a part of your fish’s coloring now.

You can get some insight into whether your fish’s black spots are due to genetics by looking at the parents. If they both have black coloring, it’s likely your fish will have some too and it’s nothing to worry about.

The important thing is to make sure the spots aren’t getting worse. If they do get worse, look patchy, or are accompanied by other signs of illness, you’ll need to investigate other possible causes.

Ammonia Burns

If you see your goldfish turning black due to chemical burns, these black spots will be irregular and patchy and usually located on the main body of your fish.

The easiest way to tell if these spots are ammonia burns is to test the chemical levels in your aquarium.

You can buy test kits online and in most pet stores. The strip tests are the easiest to use but aren’t always the most accurate. The liquid test kits are a little more complicated, but they will give you the best results and should come with instructions that will walk you through the process.

Keep in mind that black spots mean your fish has started healing, so low ammonia levels could mean your fish received chemical burns, but the ammonia has started to filter out already.

Black Spot Disease

Black spot disease should be fairly easy to rule out since it’s mostly transmitted by snails.

If you have snails in your aquarium, remove them for a few weeks and monitor your fish. If the spots start disappearing, you’ve found your culprit.

Even if you don’t knowingly keep snails with your goldfish, carefully check the rocks and decor in your tank anyway. It’s always possible you have some stowaways that hitched a ride in with your fish or other aquarium components.

If your goldfish are kept in an outdoor pond, move them to an indoor aquarium and monitor closely to see if the spots get better or worse once the fish are separated from possible parasite carriers.

Will Black Spots on my Goldfish Go Away?

If the cause of your goldfish turning black is natural or genetic, these spots are probably permanent.

If your goldfish develop black spots due to parasites, these should clear up as your goldfish heals once the parasitic life cycle is broken.

If your goldfish is turning black because of chemical burns, the spots will fade with time, provided you treat the water immediately and keep the ammonia levels down.

How To Correct The Problem

Test & Change the Water

The first step is to test the water to get a base reading of the ammonia levels.

If the levels are still high, replace at least 50% of the water. Add any necessary dechlorination chemicals or water conditioners and allow the new water to match the temperature and condition of the old water before adding it to your fish’s tank

If the ammonia levels are really high, you may need to completely replace the water.

Dose with Prime

Next, dose the freshly-changed water with Seachem Prime. This will help convert the ammonia to a non-toxic form. This is only a short-term solution, but it will protect your fish while the filtration systems remove the rest of the ammonia.

Add Carbon to the Water

Carbon is an excellent filtration additive as it neutralizes and removes many chemicals, including ammonia.

Add carbon to your tank’s filtration system or replace the old carbon if you’re already using some.


Re-Test & Change Water Regularly

It’s important to keep the tank conditions healthy so you don’t have to watch your goldfish turning black from ammonia burns again.

Re-test the water every day until the ammonia levels stay low. Be sure to change your fish’s water regularly – once a week is usually good – and re-test the levels after every change.

Limit Food

The biggest source of ammonia buildup in a tank is from rotting food.

To keep your goldfish healthy, be careful not to over-feed it. Only add as much food to the tank as your goldfish can eat at a time. If you notice an increase in leftover food floating in the water, you need to cut back on how much you are feeding your fish.

Final Thoughts

If you find yourself wondering “why is my goldfish turning black?”, don’t panic!

You may see your goldfish turning black if it’s blending in with its environment, has mixed breeding, is healing from ammonia burns, or is protecting itself from parasites.

Natural black spots can add beauty and character to your fish, and any harmful reasons behind your goldfish turning black can be usually be identified and corrected without too much difficulty.

So, don’t despair if you notice your goldfish turning black. Once you rule out any possible health problems, simply embrace and enjoy the unique and beautiful patterns that make your fish one-of-a-kind.

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