Why Is My Guppy Turning Black? {Top 4 Reasons Revealed}

Have any of your guppy fish turned black recently? Same thing happened to me and I thought, “Why is my guppy turning black?”

Are you wondering why they have changed color? The good news is they are most likely not sick.

In this article I will go through all the reasons why a guppy will turn black.

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Why Is My Guppy Turning Black?

Most of the time a guppy that has turned black is not ill. They are trying to show one of the following:

  1. Aggression: A guppy can change the color of their eyes to black to show they are feeling aggressive
  2. Stress: A guppy can show that they are stressed by changing their body to a black color
  3. Pecking Order: If new fish have just been added in to the tank the guppies will have to re-establish a pecking order
  4. Illness: Sadly an illness can also cause your guppy fish to change color. Ammonia poisoning is a leading cause.

Guppies Tail Turning Black

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Why Does My Guppy Have Black Eyes?

A guppy can change the color of their eyes to black as a sign to show aggression.  This is usually the first phase of aggression as this can alert and scare off any other fish.

Research has shown that guppies will change the color of their eyes from silver to black just before they attack another fish.

This behavior is usually only seen with larger guppies, Smaller guppies do not change the color of their eyes, most likely because they know they can not scare off other fish.

Why Is My Guppies Tail Turning Black?

If your guppies tail has turned black this is a sign of an illness. The most likely cause will be either poor water quality or an over crowded tank.

If your tank is overcrowded you should take steps to reduce the amount of fish you are keeping, or acquire a larger tank.

If you have poor water quality you will need to do the following:

  1. Change 50% of the tanks water
  2. Clean the filter in the pump
  3. Add aquarium salt (Available on amazon) to the water
  4. Add Mardel Maracyn (Available on amazon) this will kill the bad bacteria in the water
  5. Raise the temperature of the water to 80 degrees Fahrenheit.

Why Is My Guppies Tail Turning White?

If your guppies tail is turning white and showing signs of fraying then your guppy is most likely suffering with fin rot. If your guppy has fin rot you will need to take action strait away.

If fin rot is not treated it can turn in to dropsy where the guppy will start to swell up as the infection grows.

To treat fin rot in guppies I wrote this article which you can find out how to treat this condition.

Why Is My Guppy Body Turning Black?

A guppy body will change to black when they are feeling stressed, The common reason for a guppy to feel stressed is an overcrowded tank.

If your tank is over crowded you should take steps to reduce the amount of fish you are keeping, or acquire a larger tank.

Another reason could be that the guppy is breeding or spawning, During this time the guppy will not want any other fish to come close so they will change to a black color to warn other fish away.

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Why Are My Guppies Turning Black And Dying?

If your guppies have turned black and have died the most common cause is stress.  This most commonly occurs in an overcrowded tank.

If a guppy is stressed for a long amount of time this will cause their immune system to weaken and allow diseases to infect them.

Over time they will become more and more tired and the illness will grow inside them.

Why Is My Female Guppy Turning Black?

If your females guppy belly has turned back this is an indication they are pregnant and due to give birth soon.  If any other part of them has turned black it could be a sign of stress, poor water conditions or illness.

To check if your guppy is pregnant check out this guide that I wrote to find out.

However, if your tank is well kept and properly set up it can reduce the probability of your guppies getting sick. Likewise, providing your fish with the appropriate diet can also improve their immune system to these diseases. When guppies are well-fed, healthy and properly kept, they can fight off most diseases.

Disease Prevention in Guppies

It is easier to prevent diseases in guppies than allowing them to get infected before finding a cure.  You can prevent diseases from invading your tank by taking simple precautionary steps like regular maintenance and proper aquarium set up. Below are simple steps that can prevent diseases from invading your tank.

  • Do not introduce a sick fish into your tank. If you are not sure about the health status of the new fish, you can quarantine it.
  • In case of sick fish in your tank, move it to a separate tank for treatment.
  • Evacuate dead fish from the tank immediately you discover it is dead.
  • Feed your fish with quality food at regular intervals. Quality of food is more important than quantity.
  • Keep the water parameters at an optimum level.
  • Avoid the occasion of your guppies getting stressed. Increased stress level can expose your fish to the risk of being infected by diseases.
  • Carry out routine inspection on the members of your tank to be sure they are healthy.
  • Maintain regular water cleaning exercise for your tank.

Fin Rot In Guppies

Tail/fin rot in guppies is mostly caused by poor water quality. It can also be caused by bacteria that have infected a fish’s fin and tail. Likewise, this poor water quality can enhance the growth of bacteria.

The dominant symptoms of fin/tail rot are:

  • white edges
  • ragged fin and tail
  • red streaks
  • black tail
  • white-edged holes

Sometimes, the fins and tails might look like they are stuck together.

Treatment of fin or tail rot is quite easy if swift actions are taken. You can get drug medications specifically for this at your local fish pet store. All you have to do is to follow the manufacturer’s instructions for you to get effective results.


Guppy fish becoming black is mostly caused by ammonia toxicity. Genetics, stress, and black spot disease are further causes.

To keep your guppy fish from becoming black, make sure to keep the ammonia level at 0 ppm, avoid overfeeding, and give a stress-free environment.

You can save sick fish and other members of the tank by recognizing diseases and taking action early. If you have a goldfish that is turning black take a look at this article I wrote which covers all the reasons why they turn black.

Thanks for visiting HelpUsFish.com for this article. Check out our home page and search bar with hundreds of aquatic or marine life articles to choose from. Bye for now!


Hello, I'm Jason. I'm the guy behind HelpUsFish.com. I volunteer at my local fish shop and I created this site to offer tips and advice on the fish I care for.