Fish tanks require a delicately balanced environment of bacteria in order for fish to thrive. Leaving the tank dirty, using the wrong feeding or cleaning products, or purchasing fish or aquatic plants from non-reputable sellers can lead to various diseases. Let's explore some of these common pet fish diseases.

Maintaining a healthy fish tank can be a fun hobby. While some fish and fish tanks are easier to look after than others, all tanks require a level of care and maintenance to ensure that your fish don't succumb to some common illnesses.

Common pet fish diseases

Fish are often vulnerable to changes in their natural environment. While some are hardier than others, ornamental fish (those we keep in home tanks) are often most susceptible. Unbalance their environment and you could be left with an ill fish or worse, a dead fish.

A healthy fish tank environment is usually inhabited by billions of naturally occurring bacteria needed for pet fish to thrive. If you have a tank, you'll probably already know or have heard of the importance of keeping their aquatic environment in top condition.

At the same time, a poor diet could trigger some diseases in your fish. But we've got you covered!

Here is a list of the most common diseases that your pet fish could get. If your fish shares similar symptoms with any of them, visit a vet.

Vitamin deficiency

This disease is a consequence of your fish's diet. If your fish has a vitamin deficiency, you might notice how its body becomes slimmer, loses color, and your fish has breathing issues. You might also see holes in the fins and spinal deformation.

Fish that have a vitamin deficiency get tired easily. For this reason, they often stay at the bottom of their tank and try to rest.

In order to solve this issue, you should give your fish a diversified diet. This prevents your fish from deficiencies and keeps your fish healthy.


Furunculosis is a disease you will mostly find in goldfish and a few other freshwater fishes. If a fish tank has not been cleaned for a few weeks, this could cause Furunculosis. It triggers the proliferation of two bacteria: pseudomonas and Aeromonas, which are naturally present in your tank. However, if their population grows out of hand, your fish may get sick. If you want to prevent this from happening, check the nitrites and pH levels of your fish tank from time to time. You can also keep an eye on the temperature if necessary.

If your fish has rashes on its fins and muscles, your fish probably has Furunculosis. Other signs of this disease could be boils or blood clots under the fish's scales.

In order to get treated, the sick fish must be isolated from the others. The water should then be disinfected.

Fish tuberculosis

This one is caused by bacteria similar to human tuberculosis. This bacteria, however, is no threat to us. The bacteria is present in every fish but can develop any time if there is an immune deficiency. Another cause of this would be stress. Yes, it's possible.

If your fish has tuberculosis, you might notice weakness and swimming difficulties. Tuberculosis is easy to heal. Isolate your fish from the others in order to avoid any spread.

Fin rot

This disease is also linked to the quality of the environment and to several bacteria. A white-ish line appears on the fins, and shortly after they start to fray. If the fin rot is in an advanced state, the chances of surviving are really thin. For this reason, you should act as soon as possible. Once you start seeing the first signs, you should test your tank to understand what's wrong with it. After this, put your sick fish in quarantine if there are others. You can also disinfect your aquarium.

White dots disease

This one is caused by a parasite called Ichthyophthirius Multifilis. It goes through the fish's mucosa in order to set itself up. The parasite's presence can be seen with white dots on the skin. If the disease reaches an advanced state, a grey spot could appear on your fish. If not treated, your fish may not survive.

Fish mycosis

Mycosis comes from a fungus. Most of the time, injured fishes are the ones sensitive to mycosis. This is especially true after fish fights. However, the cause could also be low temperature, low levels of water, or the quality of the water. Fish mycosis should show up as a white layer on the fins, eyes, or gills, which could disappear once the fish is out of the water.


The causes are not well known yet. Sick fish have a swollen belly, due to liquid retention caused by organ dysfunction. Other signs could follow, such as scale loss, loss of control, inflammation, or even swollen eyes.

This list is short. Many other fish diseases exist. As a reminder, it is important to monitor and clean your fish tank, since it is one of the main reasons why fish get sick. Other than that, take some time to observe your pet's physical condition, and do not hesitate to ask your vet if you notice something odd. 

Originally written by Agathe Warlop. Translated by Jennifer Eubank.

Have you ever encountered any of the diseases mentioned above?

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