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Why do goldfish change color to black?

Goldfish changing color from their bright orange/red hue to a darker black shade is a relatively common occurrence for pet goldfish keepers. While some may find this color change alarming at first, there are several normal, harmless reasons why your goldfish may go from red to black over time. Understanding the various causes of goldfish turning black can help you determine if something is wrong with your fish or if it is simply a natural color shift.

Normal Causes of Goldfish Turning Black

Here are some of the most common, harmless explanations for why a goldfish may change from red to black:

  • Maturing – Young juvenile goldfish tend to be a bright, vibrant red/orange. As they grow older and mature, their coloration often darkens to a darker orange, bronze, or even black shade.
  • Genetics – Certain black goldfish varieties like Black Moors and Pandas start off silvery-gray when young then develop black coloration as they mature. Their genetic background means darker pigmentation.
  • Diet – Diets high in spirulina algae or black soldier fly larvae can enhance black pigments. Carotenoid pigments in food can also fade, letting black come through.
  • Lighting – Goldfish in tanks with low lighting and darker substrates tend to be darker in coloration overall.
  • Stress response – Prolonged stress from things like poor water quality, infections, or inadequate tank space can cause color loss including blackening.
  • Breeding seasons – Some goldfish males will temporarily turn blacker during spawning times due to hormonal changes.

Concerning Causes of Black Coloration in Goldfish

While the above reasons are generally harmless explanations for a goldfish turning black, there are some concerning causes that could indicate an underlying health problem:

  • Melanistic pigmentation – Raised black spots or patches on a goldfish can be due to an overproduction of melanin. This may be linked to health issues or genetics.
  • Ammonia burns – Prolonged exposure to high ammonia levels can damage a goldfish’s tissues and cause patchy black discoloration.
  • Nitrite poisoning – Nitrite toxicity can oxidize the blood and lead to a blackish hue, especially noticeable on the gills and fins.
  • Fin rot – Fin rot infections often cause black edges and patches on fins as the tissue deteriorates.
  • Septicemia – A systemic bacterial infection, septicemia can cause hemorrhages and dark patches from broken blood vessels and bruising.
  • Intestinal necrosis – Tissue death in the intestinal tract will release blackish compounds into the bloodstream that may cause discoloration.

If your goldfish’s black coloring seems concentrated on a certain body part like the fins or gills, or it has other accompanying symptoms like lesions, ulcers, or frayed fins, it likely indicates disease rather than a harmless natural color shift.

Diagnosing the Cause of Black Coloration

Since there are many overlapping reasons why a goldfish may turn black, it can be tricky diagnosing exactly what is causing the color change in your fish. Here are some steps you can take to get to the root of the problem:

  1. Observe where the black coloration is occurring – generalized darkening often indicates natural causes, while concentrated spots, patches, or areas imply disease.
  2. Consider timing and speed of the color change – gradual darkening over weeks/months is more likely natural versus sudden black coloring which may signal illness.
  3. Review your goldfish’s living conditions – poor water quality, crowded tanks, and insufficient filtration can lead to darkened pigmentation from chronic stress or ammonia/nitrite exposure.
  4. Examine your goldfish closely for other symptoms – look for ulcers, growths, fraying fins, sluggishness, or appetite changes that could point to disease.
  5. Have a fish veterinarian do an exam and diagnostic testing if the cause is still unclear.

Using the above steps can help you get to the bottom of why your goldfish has turned from red to black and determine if it requires treatment for disease or if it is simply natural coloration.

Preventing Concerning Black Coloration in Goldfish

While natural causes of goldfish turning black can’t always be prevented, you can take some proactive measures to reduce the chances of black pigmentation due to disease or environmental issues:

  • Maintain excellent water quality with routine testing and water changes.
  • Quarantine and treat sick fish promptly to limit disease transmission.
  • Avoid overcrowding and overload filters to prevent toxin buildups.
  • Feed a high quality, varied diet to support health.
  • Use proper acclimation procedures when introducing new goldfish.
  • Minimize handling/netting which can damage slime coats.
  • Provide an appropriately sized tank with hiding spots to reduce stress.

While you can’t fully prevent natural shifts in goldfish coloration, following best practices for goldfish care and housing can help keep concerning causes of black pigmentation to a minimum.

Treating Black Coloration Caused by Disease

If you determine that your goldfish’s black coloration is due to an underlying health condition, promptly treating the root problem is important for your fish’s welfare. Here are some treatment steps to take for common goldfish illnesses that cause black discoloration:

Disease Treatment Options
Fin Rot Antibacterial medications, improved water quality, aquarium salt baths
Septicemia Broad spectrum antibiotics, potassium permanganate dips
Ammonia/Nitrite Poisoning Immediate water change, add conditioner, improve filtration
Intestinal Necrosis Feed peas to clear blockages, antimicrobial medications

Catching and treating diseases rapidly improves outcomes. Use quarantine tanks for treatment whenever possible. Work closely with your fish veterinarian if the condition persists or worsens despite treatment.

When to Be Concerned About Black Coloration

As a general rule, you should be concerned about your goldfish turning black if you observe:

  • Sudden onset of black coloring versus gradual darkening over time
  • Dark patches or spots rather than an overall color change
  • Areas of black concentrated on certain body parts like fins or gills
  • The black color persists or worsens despite improving conditions
  • Other symptoms present like lesions, appetite loss, respiratory distress

While some darkening of color can be expected as goldfish mature, localized black spots or rapid pigment change often indicates disease. Seek veterinary advice if you are worried.

Is Black Coloration Permanent in Goldfish?

Whether goldfish black coloring is reversible or permanent depends on the underlying cause:

  • Natural causes of black pigmentation like aging, genetics, or diet typically cannot be reversed.
  • Blackening due to poor water quality will often resolve with improved conditions.
  • Disease-related dark spots or patches may resolve with prompt treatment.
  • Long-standing illnesses can result in permanent scarring and discoloration.

So while black coloring due to disease can sometimes be reversed if caught early, natural shifts in pigmentation are usually permanent. Regardless of reversibility, addressing the root cause is key.

Should You Be Trying to Reverse Black Coloration?

If your goldfish develops black pigmentation without any other signs of illness, it is generally not problematic and requires no corrective action. Natural color changes are a normal part of maturation and genetics.

However, if there are indicators the blackening is due to disease or environmental issues, take steps to remedy the root problem through water quality improvements, isolation of sick fish, and appropriate medical treatment. The black coloring itself is not the issue if it is not associated with disease.

Focus on ruling out concerning causes and providing good general fish care rather than trying to reverse harmless black pigmentation itself. Darker coloration alone is not dangerous if the fish is otherwise healthy.

Caring for Black Goldfish

Caring for black goldfish is the same as caring for red/orange varieties. Here are some key goldfish care guidelines:

  • House in at least a 20 gallon tank with efficient filtration.
  • Maintain water parameters within safe ranges (ammonia and nitrite at 0 ppm, nitrate
  • Perform 25% weekly water changes and deep cleaning monthly.
  • Feed a varied diet of flakes, pellets, and treats.
  • Keep tank temp between 65-75°F.
  • Use proper acclimation for new fish and quarantine as needed.
  • Watch for signs of disease and treat promptly.

Goldfish are social fish best kept in groups. Ensure all tankmates are peaceful, compatible fish with similar care needs. With good husbandry, black goldfish can live just as long and healthy lives as red/orange varieties.


Goldfish changing from red to black can be alarming for owners at first glance. However, it is often a harmless natural process related to maturation, genetics, or environment. Differentiating between benign causes of darkening versus disease-associated pigmentation is key to determining if action is required.

While long-term black coloration may not be reversible in many cases, focusing on addressing the underlying issue through water quality improvements, disease treatment, reduced stressors, and good nutrition can help impacted goldfish return to health. With proper care, black goldfish can lead full, high-quality lives.