Guppy Black Spot Disease – Causes And How To Treat 

Black spots will ruin the stunning look of your guppies. They also irritate their body and damage their health over time. So how can you deal with this disease?

We have received a lot of questions about guppy black spot disease from fish keepers. So today, we will show you some tips to identify the spots on your fish and how to treat them properly. Let’s read on to discover! 

Guppy Black Spot Disease  

Black spot is a disease caused by Turbellaria flatworms. These worms, like other parasites, need a host to live on. 

The young Paravortex starts feeding on the host’s cell fluids as soon as they locate a fish by tunneling into the epithelium of the host’s gills or skin. 

These parasites are far less dangerous than protozoan parasites. Moreover, you can identify and treat it easier. Yet, you still need to cure it as soon as possible because it’s unattractive and damaging to the guppy’s overall health. 

We will discuss the disease’s symptoms, causes, and proper treatment. Please check and apply the guidelines to your pets. 

Guppy black spot disease


Apparent signs of the black spot disease include: 

  • Black spots on the guppy’s body
  • Scratching or rubbing against other objects
  • Decrease or loss in appetite
  • Lethargy
  • Faded color

These worms develop melanin coloration after they feed on the host. Then, you can see black spots that are as big as a grain of salt on the fish’s fin and skin. 

The worms can move freely on the fish so that the spots won’t remain in one position. You will notice the spots on light-colored fish easier. 

Moreover, the infected guppies may scratch against aquarium objects as they try to remove the surface parasites. 

The fish may lose their appetite, become sluggish, and lose color if the infection spreads. The infected areas are the gills, and you will see more rapid respiration. 

Check for black spots on your fish


Turbellarian flatworms, often Paravortex, cause all the black spot problems. They belong to the Paravortex species group. 

Although this disease is not extremely common in home aquariums, its rapid reproduction speed will be a headache. It can spread quickly throughout your tank without prompt remedy. 


Before treating, make sure that the disease your pets suffer is the black spot. The signs can tell, but you should contact an aquatic veterinarian for the exact diagnosis. 

Removing the gravel from the aquarium during infection may help minimize encysted worms. You must isolate the infected fish to stop the parasite community in the main tank from growing. 

Then, give the sick guppies a water dip and a formalin bath. Try to stick to the product’s instructions when mixing and applying them. 

You can also use praziquantel to cure the isolated guppies. Praziquantel is a medication that can treat some parasitic worm infections in fish. 

There are several ways to save your fish

How to Prevent a Guppy Black Spot? 

No matter how well you cure your fish, reinfection will happen if parasitic worms are still in the main tank. They can live without a fish host for a few months. Here are things to do about this problem. 

Leave the main tank empty for a while 

Some aquarists suggest removing all the fish from the main tank and leaving it empty for a few months. The idea is to wait for the life cycle chain of the worms to break down, and they can’t reproduce anymore. 

Clear the debris and waste 

Young worms rely on organic waste and debris on the substrate where they live before looking for a fish host.

You can control their population by siphoning off the extra organic debris that accumulates on the tank’s bottom.

You can use hyposalinity (lower salt content) to eliminate the worms if you have a fish-only aquarium. Make sure there aren’t any freshwater-sensitive crustaceans. 

Remember to stir the substrate to release the salinity water stuck in the gravel. This method will expose the worms to the salinity of the water content in the tank. 

Clean the tank regularly to eliminate the harmful worms


1. How do you know when guppies are sick?

To diagnose your guppy, you must be aware of the symptoms of a sick fish, such as:

  • Spots on the fish’s body
  • Decrease or loss in appetite
  • Swollen bodies 
  • Discoloration 

This video will give you more information about fish’s health requirements: 

2. How do fish get black spot disease?

The Turbellaria flatworm’s reproductive cyst sits in the tank substrate. The young worms can move freely after spawning, so they quit the substrate searching for a host fish.

The young Paravortex enters the fish’s gill or skin and starts feeding on the cells once they have found a suitable host, causing the disease. 

3. Why do guppies turn black?

Ammonia poisoning is the most common cause of guppies turning black. If it’s not your case, your fish may be suffering from black spot disease, stress, or genetic issues.  

4. Is black spot disease contagious to other fish? 

Yes. If not treated soon, the disease will spread to other fish in the tank and damage their cells. Once you notice any infected fish, isolate them to avoid the spread. 

5. How often should you change water for guppies?

It would be best to clean your fish tank once a week and change about 25% of the water.  


Flatworms cause black spots on guppies. Luckily, the signs of this disease are easy to notice. With prompt treatment, your fish can be healthy again.

Hopefully, we have satisfactorily addressed your problem. If you have any questions, please feel free to ask. We will cover it in the next post.

Thank you for reading!