Guppy's Black Patches: Why?

why does my guppy have a black patches

Guppies are fascinating creatures, but sometimes they can exhibit strange behaviours or physical changes that may cause concern for their owners. One such change is the appearance of black patches on the fish's body. While this can be alarming, there are several possible explanations for this phenomenon.

One common reason for black patches on guppies is aggression. Guppies will sometimes blacken their eyes as a way to establish dominance and hierarchy within their group. This behaviour is often seen before breeding or giving birth, as well as when protecting food sources. By turning their eyes black, guppies can warn potential rivals to stay away and signal their willingness to attack if threatened.

In addition to aggression, black patches on guppies can also be indicative of an infection. Guppies are susceptible to various viruses, parasites, and bacteria that can cause darkening of the tail or spots on the body. For example, black spot disease, caused by parasitic flatworms, can result in black spots on the flesh of guppies.

Genetics also play a role in guppy coloration. Specific genes determine their colour patterns, and selective breeding for certain traits can influence the prevalence of black patches. In some cases, guppies may develop dark patches simply due to their natural coloration and genetics, rather than any external factors.

Another possible explanation for black patches on guppies is stress. A stressful environment, such as overcrowding, poor water quality, or the presence of aggressive fish, can trigger colour changes, including the appearance of black patches. Stress can also lead to other behavioural changes such as loss of appetite and antisocial behaviour.

While black patches on guppies can have various causes, it is important for owners to monitor their fish closely and seek veterinary advice if they suspect any health issues. By understanding the potential reasons behind this colour change, guppy owners can take appropriate steps to ensure the health and well-being of their aquatic companions.

Characteristics Values
Aggression Guppies turn their eyes black to express aggression and to intimidate other fish.
Infection Infections, including viruses, parasites, and bacteria, can cause black spots or patches on the guppy's body or tail.
Stress A stressful environment, such as overcrowding, poor water quality, or the presence of predators, can trigger blackening.
Genetics Genetics can influence the coloration of guppies, especially if they are selectively bred for certain traits.
Pregnancy A black spot on a female guppy's belly is an indicator that she is pregnant.


Guppy genetics

Basic Genetics

Guppies inherit two copies of each gene, one from each parent. Some genes are dominant, meaning their presence in one copy is enough to express the trait. Other genes are recessive, requiring both copies to be present for the trait to manifest.

Guppy Colouration

Guppies come in a variety of colours and patterns, with different combinations of pigments. The most common varieties include:

  • Metallic guppies: These have reflective platelets in their skin, giving them a metallic sheen. Common colours include gold, silver, and platinum.
  • Solid-colour guppies: These have a single base colour throughout their body without patterns or markings. Examples include red, blue, or yellow guppies.
  • Bi-coloured guppies: These feature two distinct colours, often with a clear separation line along their bodies. Combinations can include black and orange or blue and yellow.
  • Tuxedo guppies: These have a solid body colour with contrasting tails and fins, resembling a tuxedo.
  • Snakeskin guppies: These have a scale pattern resembling snake scales, usually with a base colour.
  • Leopard guppies: These have small, dark spots or splashes, similar to a leopard's coat, of various colours and sizes.

Guppy Breeding

Breeding guppies allows for the development of specific colour patterns and the enhancement of desirable traits. Guppy genetics can be complex, and creating a new strain requires time and energy. When breeding guppies, it is essential to consider the inheritance patterns of specific traits, including dominant and recessive genes.

Guppy Colour and Environment

The substrate, or tank bottom, can also impact guppy colouration. Darker substrates like black or deep brown can provide a contrasting backdrop that highlights the vibrant colours of the guppies. Additionally, fine gravel or sand substrates are ideal as they mimic the natural environment of guppies, providing an optimal environment for their health and well-being.

Black Patches on Guppies

Now, to address the black patches on your guppy, there could be several reasons for this colouration:

  • Aggression: Guppies may darken their eyes, turning them black, as a sign of aggression or to establish dominance.
  • Infection: A black tail on your guppy may indicate an infection by viruses, parasites, or bacteria. However, it could also be a natural development as they grow, depending on their genetics.
  • Parasitic flatworms: Black spots on your guppy may be caused by black spot disease, which is attributed to parasitic flatworms.
  • Ammonia: High ammonia concentrations in the tank can damage your guppy's gills and restrict their breathing, leading to red streaks and black patches on their body.
  • Stress: A stressful environment, such as overcrowding, aggressive tankmates, or poor water quality, can cause drastic colour changes, including black shades.
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Guppies are generally peaceful and social fish, but they can become aggressive under certain conditions. Aggression in guppies can be caused by sexual competition, overcrowding, food scarcity, a lack of hiding spots, and a small group size.

Sexual competition, for instance, can cause male guppies to become more aggressive than females as they compete for mates. This can lead to chasing, nipping, and fin display behaviours to impress females and intimidate other males. Overcrowding can also trigger aggression and territorial behaviour as guppies defend limited resources and space.

Guppies use blackening their eyes as a way to express aggression and establish dominance and hierarchy, warning potential rivals to stay away. This behaviour is particularly noticeable when guppies are about to breed or give birth. The blackening of the eyes is a result of the dark pigment cells in their irises, which can be controlled by their nervous systems.

To prevent and reduce aggression in guppies, it is important to maintain a proper male-to-female ratio, provide enough space and optimal water quality, offer a varied and sufficient diet, and add hiding spots and decorations to the tank. Keeping guppies in a large and compatible group can also help prevent aggression.



Guppies are susceptible to a variety of infections, including viruses, parasites, and bacteria. Here are some common infections that can cause black patches on your guppy:

Black Spot Disease

Black spot disease is caused by parasitic flatworms that initially infect birds. The parasites mature in the intestines of birds, which then release the parasite eggs into the water through their droppings. These parasites can appear on the flesh of guppies as black spots and cause them to scratch against objects in the tank. This disease can also lead to respiratory distress, making the guppies gasp for air.

Ammonia Poisoning

High levels of ammonia in the tank can be toxic to guppies and can cause black patches on their bodies. Ammonia is produced when leftover fish food and dead plants rot, leading to elevated pH levels. Ammonia can damage the gills and restrict the guppy's ability to breathe, causing them to gasping for air and lose their appetite.

Guppy Disease (Tetrahymena)

Guppy disease is caused by ciliate protozoans in the genus Tetrahymena. It resembles ich or white spot disease, with affected guppies exhibiting white spot-like cysts, heavy breathing, lethargy, and clamped fins. Guppy disease can progress rapidly, and infected guppies may appear healthy one day and die the next. It is challenging to treat, and medications for ich or velvet are typically ineffective against Tetrahymena.

Fin Rot

Fin rot is a bacterial or fungal infection that can cause the fins and tail of guppies to rot and stick together. Poor water quality and ammonia burns are common causes of fin rot. It is important to determine the cause of the rot, as bacterial and fungal infections require different treatments.

Protozoan Infection

Protozoan parasites attach to the skin of guppies and slowly enter their bodies through the muscles, eventually reaching the bloodstream. This infection typically occurs in unheated tanks with poor water quality. Early-stage treatments include Malachite Green or Formalin, while advanced stages may require copper medication.

Other Infections

Guppies are also susceptible to a range of other infections, such as Columnaris, Mouth Fungus, Dropsy, Gill Flukes, Camallanus Internal Worm, and Hexamitiasis. These infections can cause various symptoms, including swollen gills, gasping for air, loss of appetite, discolored skin, and abnormal behaviour.

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A stressful environment can cause guppies to change colour drastically, including turning black. Stress in guppies can be caused by several factors, including:

  • Overcrowding
  • The presence of aggressive fish
  • Incorrect water temperature
  • Dirty water

To alleviate stress in guppies, it is important to ensure that their tank mimics their natural environment. Guppies are usually nestled amidst plants, tree roots, branches, and other natural hideaways, so adding live aquarium plants can help create a more comfortable environment for them. Overcrowding should also be avoided, as it is another significant stressor for guppies.

Temperature fluctuations can also cause stress in guppies, so it is important to use a heater to maintain a stable water temperature. Regularly performing partial water changes of 30-50% each week and ensuring that the new water matches the desired pH level and temperature is crucial.

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Ammonia poisoning

Ammonia is produced in a fish tank by the decomposition of organic matter, such as uneaten food and dead plants, as well as the waste produced by fish. Ammonia levels can also rise due to the use of chemically treated tap water, a buildup of bacteria, or the introduction of new fish. Elevated ammonia levels can cause the pH levels in the tank to rise, damaging the gills of the fish and restricting their ability to breathe.

The symptoms of ammonia poisoning in fish include gasping for air at the surface of the water, loss of appetite, lethargy, red or purple gills, and bloody patches on the body. As the poisoning progresses, the fish's tissues will deteriorate, and the fish will eventually die.

To treat ammonia poisoning, it is important to act quickly. Lowering the pH of the water and performing a partial water change can provide immediate relief. In severe cases, a chemical pH control product may be necessary to neutralise the ammonia. It is also important to restrict feedings to reduce waste and, in very high ammonia levels, discontinue feedings for several days.

To prevent ammonia poisoning, it is important to maintain a clean tank, remove any dead plants or uneaten food, and perform regular water changes. It is also crucial to test the water for ammonia levels regularly and to avoid overstocking the tank with too many fish.

Frequently asked questions

Guppies can develop black patches due to various reasons, such as aggression, infection, stress, or genetics. Aggressive guppies may intimidate other fish by blackening their eyes. Infections by viruses, parasites, or bacteria can cause black spots or patches on the guppy's body or tail. Stress from factors like overcrowding, poor water quality, or the presence of predators can also trigger blackening. Additionally, genetics can influence the coloration of guppies, especially if they are selectively bred for certain traits.

To prevent black patches, it is important to maintain optimal water quality and reduce stress levels in the tank. Ensure proper water parameters, provide hiding spots, and avoid overfeeding and overcrowding. To treat black patches, identify the underlying cause and take appropriate action. For example, if aggression is the issue, adjust the male-to-female ratio and provide sufficient space. If infection is the cause, consult an aquatic veterinarian and follow their recommendations for treatment and quarantine procedures.

Black patches on guppies can be a result of various factors, and their impact depends on the specific cause. For example, black eyes due to aggression are typically not harmful, but black spots or patches caused by infections or parasites may require veterinary attention and treatment. It is important to monitor your guppies' behavior, appetite, and overall health to determine if the black patches are accompanied by any other symptoms that may indicate a more serious issue.

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