Why is My Guppy Turning Black? (With Solutions)

Guppies are among my favorite types of fish. However, more than once, changes in their behavior and appearance got me worried. There were times when I even saw my guppy fish turning black. That issue got me so disturbed that I began researching the topic pretty intensively.

Guppy fish turn black to express aggression. Blackening their eyes allows them to establish dominance and hierarchy, eliminating possible threats. However, guppies also become black due to infections, including viruses, parasites, and bacteria.

As we move forward, I will show you how to determine whether your guppy is sick or merely presenting signs of aggression. I will also elaborate on how to treat guppies that had turned black, and how to prevent that worrisome phenomenon.

Why do Guppies Turn Black? (Eyes, Tails, Black Spots & More)

Goldfish can turn black; it isn’t unheard of. It isn’t necessarily a positive occurrence, either. But many people have seen black coloring in goldfish, which is why it doesn’t always elicit surprise.[1] Nevertheless, guppies are a different issue.

They are not necessarily known for exhibiting black colorations. As such, you have every reason to worry when you notice this development. Still, you cannot treat or reverse this coloration without first understanding the cause.

The causes of a guppy turning black will vary widely. Your ability to diagnose their origins will depend on the exact nature of the black coloration. This is what you can expect:

1. Aggression (Black Eyes)

That particular case shouldn’t concern you. Guppies have silver eyes. But those eyes can turn black when they get aggressive.[2] Yet, this attribute might catch some aquarists off guard, and no one would blame them for panicking in such a situation.

But black eyes in guppies are entirely natural. They use this transformation to warn potential rivals to stay away. For instance, back eyes will manifest shortly before a guppy attacks an enemy.

This works in your favor because it will immediately authenticate the black eyes in your guppies as a sign of aggression rather than a symptom of a disease. It will even become more pronounced when your guppies are about to breed or give birth.

2. Infection (Black Tail)

If your guppy’s tail is turning black, it is probably sick. Guppies are strong. However, they are still susceptible to numerous viruses, parasites, and bacteria, some of which can produce unexpected symptoms, including a blackening tail.

Nevertheless, you should first make sure that genetics aren’t involved. Some guppies merely develop dark tails as they grow, as a simple matter of coloration. Still, if the guppy’s parents didn’t have this feature, an infection is the most probable cause.

3. Parasitic Flatworms (Black Spots)

Is your fish turning black all over, or is it merely developing black spots? The two attributes are different. Black spots are hard to miss. They are caused by black spot disease, which is attributed to parasitic flatworms.[3] When they infect a guppy, they can appear on the flesh of the creatures as black spots.

Black spot disease is challenging to control. The parasites that cause it initially infect birds. After maturing in the intestines, the birds introduce the parasite’s eggs to the water through their droppings. They penetrate snails from where they undergo additional development before taking flight and finding their way into the skin of fish.

If your fish has these parasites, you will see it scratch against the objects in the tank. Fish are no different from humans when they itch – they need to scratch. Black spot disease can also cause guppies to gasp for air because they are struggling to breathe.

4. Ammonia (Black Patches)

High ammonia concentrations are inadequate for guppies. High levels of the substance can kill your fish. Ammonia manifests when leftover fish food and dead plants rot. As a consequence, it will elevate the pH levels in the tank.

It can also damage your fish’s gills, restricting their ability to breathe.[4] Tanks with high concentrations of ammonia will cause your fish to gasp for air. You will see them on the surface of the aquarium. They will also lose their appetites before becoming lethargic.

Ammonia poisoning can affect the physical appearance of your guppy. It damages the tissues of the fish, causing red streaks to appear on its body. Sometimes, this can produce black patches. 

Some people have argued that ammonia burns the fish, which is why their bodies change color. If you suspect this is your case, I highly suggest reading an article I wrote on why do fish rapidly move their mouths. I made sure to include there a solution for the ammonia issue.

5. Stressful Environment (Variable Blackening)

A stressful environment can force drastic color changes to occur in your fish, including black shades. Stress has several causes, including overcrowding, the presence of aggressive fish, the wrong temperature, and dirty water. 

Besides the change in coloration, stress is accompanied by additional signs such as a loss of appetite and antisocial behavior. Some unhappy fish spend a lot of time hiding, especially if their anxiety comes from a bully in the tank.

How to Treat a Guppy That Has Turned Black?

It is worth reiterating the fact that a black coloration is not natural in guppies (besides eyes blackening). Therefore, once you realize that your guppies are turning black, you must take immediate action to treat them. 

Sometimes, this is the only way to save your guppy’s life. Some options at your disposal include:

1. Consult a Vet

Talk to a medical professional, especially if you are new to guppies. If you don’t know what to make of the black coloration and notice additional symptoms such as gasping and lethargy, consult a veterinarian. The earlier, the better. 

A fish vet can identify all the potential viruses, parasites, and bacteria responsible for the change in color. Your best choice is bringing your sick guppy to experts. They will be able to identify diseases such as black spots disease and treat them accordingly.

2. Maintain Proper Ammonia Levels

You don’t have to wait for your fish to display additional symptoms to determine that ammonia poisoning is the cause of their ailment. You can just test the water. To do that, I highly recommend checking the API Reef Master Test Kit (link to Marine Depot).

Once you realize that the concentration is too high, use chemicals like chloramine that bond to ammonia. But treating the water isn’t enough. You must also identify any tank elements responsible for elevating the ammonia concentrations, including the presence of uneaten fish food, dead plants, and fish.

You must remove these sources of corruption. Otherwise, they will simply undo the results of the chloramine treatment before poisoning your fish once more. Make sure you properly clean the tank, removing contaminated debris.

Where the guppies are concerned, you can immediately relieve the effects of the ammonia by lowering the pH. The best way to do that is by performing a partial water change. The general rule of thumb is to replace a third of your tank once a week. 

However, if the case is severe and the distress in your guppies is not subsiding, use chemical products to eliminate the ammonia. Given enough time, so long as you maintain a pristine tank, your guppy will gradually recover.

3. Eliminate Stress

If stress is the cause of the black coloration in your guppies, find the sources, and eliminate them. You can start by testing the water parameters to ensure that the pH, temperature, nitrate, and nitrite levels are appropriate.[5] If they are not, take steps to alter them. 

The wrong pH and temperature can cause severe distress in fish. Ensure the temperature is between 74 and 82 degrees Fahrenheit.[6] For that, I highly recommend checking my aquarium heater review. That is the only device that keeps my water stable, with such minimal temperature fluctuations. 

If aggressive tankmates are to blame, move them to another tank. You can also add an aquarium divider, as was described in the Youtube video above. If overcrowding is the cause, try getting a bigger tank. 

Also, make sure that you keep the tank in a quiet location, away from human traffic. Don’t permit people to tap on the glass, startling your fish. Create a conducive environment that will alleviate the stress in your guppies.

4. Feed Your Guppies Adequately

Keep your guppies adequately fed. A suitable diet is the best medicine in some cases. The right diet will ensure that your guppies have all the nutrients they need to keep infections and diseases at bay. Try maintaining a regular schedule.

In general, you should feed your guppies twice a day. Also, provide a mix of flakes and pellets, live foods, and freeze-dried foods. If your guppies are not eating in sufficient quantities, use a turkey baster to spray the food directly into the areas they frequent. 

A well-fed fish is more likely to overcome parasites and bacteria. A starving fish, on the other hand, is vulnerable. However, properly feeding your guppies is merely an additive measure. You should also implement the steps I mentioned earlier.

Related article that may also interest you:

How to Prevent Your Guppies From Turning Black?

It is easier to prevent your guppies from turning black than it is to treat them once they’re afflicted. The following tips will also help you in case you’ve solved the blackening guppy issue, and you wish to avoid reoccurrence:

1. Choose New Tankmates Wisely

Pay close attention to the creatures you introduce to the tank, avoiding infected fish. Sick guppies are not that hard to identify. Look for clamped fins, white spots, fuzzy patches, cloudy eyes, bulging eyes, deformed spines, swollen or sunken bellies, and lethargy, to mention but a few.[7]

Many fish stores have no qualms about selling sick fish to unsuspecting customers. This is why you must do your due diligence. Scrutinize each fish. Keep it in quarantine to ensure that it is healthy and free of diseases before you add it to your guppy tank.

2. Move Infected Fish to Quarantine

You should isolate the fish in question once you observe any of the signs of sickness mentioned above. It is rarely a good idea to permit sick guppies to continue mingling with other fish. Even if you have already started treating them, do this in a separate tank. 

Otherwise, they will spread whatever they have to your healthy guppies. Then, observe the rest of your fish daily. Make sure that they don’t develop any symptoms like the infected guppies. If they do, move them to quarantine as well.

3. Eliminate Dead Fish

Remove dead fish the moment you see them. This also applies to dead plants. Basically, remove anything that can rot; that includes uneaten food. Check the substrate regularly (this is where leftovers typically settle).

Make sure that rotten food or fish have not blocked your filter’s circulation. Disassemble your device and wash it thoroughly under running water. The sponge is usually a problematic area. Make sure that it is entirely clean before putting it back in.

4. Avoid Overfeeding

Don’t overfeed your guppies. First of all, they will allow some of the excess food to go uneaten, and it will rot, creating the ammonia problem. Secondly, overfeeding will make your guppies sick, possibly even constipated. 

Additionally, overfed guppies will generate more waste. That, in turn, will impact the pH and turn the tank much harder to maintain. Generally, you should feed your guppies the amount of food they typically consume within three minutes.

5. Pick The Right Equipment

Get a reliable, properly working filter. However, it shouldn’t be so strong that it overwhelms your guppies with its current. But it should be strong enough to keep the tank clean. You should also add heaters that will maintain the appropriate temperature in your tank.

Keep all your testing tools on hand. Don’t wait for your fish to show signs of distress before acting. Test the water for ammonia, nitrites, nitrates, pH, and the temperature to ensure that all the parameters are within the desired range.

6. Maintain Hygiene

It isn’t always enough to use a filter. Besides cleaning the tank walls, removing algae, and eliminating dead plants, dead fish, and uneaten food, you should carry out regular water changes. This is the only way to maintain the hygiene of your tank, not to mention keeping all the parameters within the proper range. 

Also, you should inspect the tank routinely. Study every element in the aquarium to ensure that your guppies have all the features needed to live healthy lives. If you have doubts, consult a doctor. Do not act on wild guesses. If you don’t have the answers, ask for help.


Guppies tend to become black when they are stressed. That is primarily manifested in their eyes, as they become darker to scare away predators. You may have noticed that behavior in an overcrowded tank, or possibly when breeding and spawning is nearby.

Nevertheless, guppies also turn black when they are sick. In that case, you may see that their tails have become darker, or that black spots have appeared. If that is your situation, your best next step is seeking for a vet. 

Meanwhile, make sure that the infected guppy is in quarantine and that the community tank conditions are proper. That primarily includes pH, temperature, and ammonia concentrations.


  1. https://puregoldfish.com/turning-black/
  2. https://www.sciencemag.org/news/2018/06/when-they-re-attack-guppies-eyes-turn-black#:~:text=That’s%20because%20guppy%20irises%20turn,get%20aggressive%2C%20The%20Atlantic%20reports.
  3. https://www.in.gov/dnr/fishwild/3395.htm
  4. https://www.thesprucepets.com/ammonia-poisoning-1378479
  5. https://www.reference.com/pets-animals/fish-turning-black-df176f0bf4d3ead7
  6. https://pets.thenest.com/temperature-should-aquarium-guppies-12350.html
  7. https://animals.mom.me/guppy-fish-healthy-4486.html

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