Goldfish are one of the most popular pets in the world! They’re beautiful, hardy, relatively easy to care for, and can be pretty interactive with their owners.
So, if your brightly coloured pet goldfish starts turning black, you want to know why.
Based on my 40 years of experience in goldfish keeping, I can reassure you that a colour change in your goldfish can be a natural phenomenon and is not necessarily all bad. However, there are several sinister reasons why a goldfish might turn black, and as a responsible fish owner, you should know them.
Read this guide to learn why your goldfish might turn black and get some tips on treating and preventing it.
Should goldfish change colour?
Juvenile goldfish are not born with their true adult colours but develop them later in life as adults. In addition, sometimes, mature goldfish change colour several times during their lives. For example, my gorgeous silver and orange Ryukin fancy goldfish gradually lost all its vibrant orange colours, ending up completely silver.
Goldfish colour is influenced strongly by their environment, starting with the amount of light in the habitat. For example, goldfish born in a brightly lit environment have more vibrant colours than those that live in a deep pond with dim lighting.
Sometimes, feeding your goldfish a type of food containing carotene can help to make an orange or red fish’s colours more vibrant. However, a fish’s natural coloration is more dependent on its genetics than anything else.
Do goldfish colours change during breeding?
Some owners report seeing colour changes in their goldfish during spawning. Typically, male fish develop brighter, deeper colours when in breeding condition, although I can’t say I noticed that phenomenon in my fish.
Why is my goldfish turning black?
Black is a highly unstable colour in fish. In fact, many black goldfish gradually fade to yellow, bronze, or even white, and seeing a healthy goldfish become darker is extremely rare.
There are several reasons why your goldfish might start turning black, and I explain them in the following part of this guide.
Ammonia in the water
By far, the most common reason for goldfish turning black is the presence of ammonia in the water.
The colour change starts on the fins, progressively worsening until it spreads over the unfortunate fish’s whole body, appearing as small patches of burnt-looking black skin. Actually, those black patches are scarring and part of the healing process. However, there are clearly issues with the water chemistry in your aquarium that you must address.
Of course, if your tank is properly cycled and well-maintained, and you run an efficient filtration system, the water shouldn’t contain any ammonia at all! Ammonia is a toxic chemical that’s produced when organic matter, including fish waste, dead plants, and uneaten fish food, decays.
You can get rid of the chemical and keep it out of your tank by performing partial weekly water changes, removing organic waste matter from the substrate with an aquarium vacuum cleaner, and ensuring your filter media is clean and sludge-free. A clean tank should have zero ammonia in the water.
Even a tiny amount of ammonia can kill your fish. In fact, a concentration of just two parts per million will be fatal. So, ideally, you always want to have zero ammonia in your goldfish tank.
Ammonia burns the goldfish’s skin, and when the chemical is present in higher concentrations, it burns the sensitive tissues of the fish’s gills, effectively suffocating it.
However, the good news for your fish is that those black patches are signs of ammonia damage healing like scars on human skin. That could indicate that levels of the chemical have dropped to a safe level again, possibly following a tank deep clean or filter media replacement.
But assume nothing! Test the aquarium water with an aquarium testing kit once a week to be sure that the water chemistry is correct and ammonia levels are zero.
Some varieties of goldfish are genetically more likely to change colour than others.
That said, it’s pretty rare for a goldfish to turn black, but it is still possible. I’ve had several goldfish that became paler as they aged, but never one that turned black!
All metallic varieties of goldfish start off silver grey, turn black, and then slowly fade to a light yellow before their colour deepens to orange or red, sometimes with white, silver, or blue patches. Metallic goldfish typically start changing colour within three months of hatching.
In a nutshell, healthy goldfish routinely change colour as they age. That change should be gradual, meaning any sudden colour changes can be a cause for concern.
Black spot disease is more commonly seen in pond-kept goldfish than those living in the more regulated environment of a fish tank. In fact, black spot disease affects most wild fish species, and fishermen angling in wild habitats often encounter it.
The condition is caused by a parasitic flatworm that infected aquatic snails carry. Birds eat the snails, carrying the disease and passing it onto pond-kept goldfish in their droppings. Black spot disease causes the fish to develop varying amounts of black spots across their bodies, and serious infections result in so many black spots the fish appears to turn black.
The black spots are actually encysted eggs, incubating in the fish’s skin until they erupt to release the next generation of parasites. The burrowing activity of the parasites and the eggs themselves cause skin irritation, so if you spot your goldfish flicking or rubbing on objects in the tank, that’s a sign that this disease is the culprit for your fish turning black.
Remove any snails in the tank or pond to get rid of the disease, breaking the parasite’s life cycle and eventually eradicating it. Although it can take time, eventually, the fish will recover. However, I would stress that tank-kept goldfish rarely suffer from black spot disease, and it’s usually only a problem for fish living in garden ponds.
What to do if your goldfish is turning black
The first thing to do if your goldfish is turning black is to check the water quality in your fish tank. That’s essential if your pet exhibits other symptoms, especially the following:
- Sitting on the bottom of the tank
- Laboured breathing
- Not eating
- Reddened gills
All those signs can be indicative of ammonia poisoning.
Immediately, perform a 20% partial water change using an aquarium vacuum cleaner to remove organic waste from the substrate and around plant bases. Check your filter media, and clean or replace it if it’s spent. Retest the water and carry out further water changes if necessary until the ammonia level is zero.
Once the water quality is improved, your fish should begin to recover.
If your fish develop black spots and are flashing or flicking against solid surfaces in the tank, black spot disease is likely the culprit. Remove any snails from the tank and monitor the goldfish. They should recover once the remaining parasites have hatched and died off naturally.
Will black spots on a goldfish go away on their own?
Genetic black spots won’t disappear unless the fish’s colours fade naturally.
Black blotches due to injury will disappear once the fish’s skin heals, and shed scales often regrow. A fish infected with black spot disease will lose its black coloration once the parasites have hatched and died off.
Why is only one goldfish turning black?
There are several reasons why only one goldfish turns black while its tank mates retain their usual bright hues.
One possibility is that those black colours are changing naturally because of the fish’s genetics or the ageing process. Remember that the goldfish you buy at your local fish store are likely from mixed sources, so they’re genetic makeup is probably quite different.
Sickness or stress-related illness can cause colour changes if only one fish is affected.
Can goldfish be black or turn black naturally?
As mentioned earlier, goldfish coloration often changes to some extent throughout the creature’s life. So, if that colour change includes black splotches and markings, that’s not a cause for concern.
There are some goldfish varieties that are meant to be primarily black in colour, including Black Moors. In addition, calico goldfish have black markings as part of their unique patterns and colours.
Will my goldfish stay black once it has turned black?
Once your goldfish has turned black, it might not necessarily stay that way.
As explained in this article, there are several causes for colour changes in goldfish. Black patches caused by disease-related scarring or infection often disappear as the fish’s skin heals, provided the root cause of the disease or problem is addressed.
However, as I discovered with my goldfish, their coloration often continues to evolve and change throughout their lives, typically becoming paler rather than darker as the fish ages.
There are several reasons why your goldfish is turning black.
Goldfish do tend to change colour throughout their lifetime, although they generally get paler, not darker. Some varieties of goldfish, like Black Moors, are supposed to be black, although some are more of a dark bronze shade than true black. Also, calico-coloured goldfish and Shubunkins should have black markings that often change over time.
However, if your goldfish appears unwell, ammonia poisoning is the primary reason for it to turn black, and black spots on the fish’s body could indicate black spot disease. Reassuringly, once the disease or water quality issues have been rectified, the black coloration should eventually disappear.