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Main Diseases of Aquarium Fish

Main Diseases of Aquarium Fish

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Contemplating an aquarium soothes the mind and lowers blood pressure. Beyond these benefits, regular and detailed observation of the inhabitants of your aquarium is an integral part of maintenance operations. It is important to ensure the well-being and health of the ecosystem you have created at home.

Diseases are common and can quickly cause significant damage. It is not only your wallet that is at stake, but also the lives of your fish. The information provided here will help you identify diseases. We do not provide treatment methods as it is a complex subject, and some interventions can be intricate.

Contagious Fish Diseases

Contagious diseases are the most problematic as a single individual can quickly infect all the others. Once the disease is identified, it is advisable to isolate the diseased fish in a separate “hospital” aquarium.


Ichthyophthiriasis, commonly known as “white spot disease,” is one of the most common diseases in aquarium fish. It is caused by the parasite Ichthyophthirius multifiliis, which is frequently found in freshwater aquariums. Its marine equivalent is Cryptocaryon Irritans.

The infection is facilitated by conditions that weaken the fish and make them more susceptible to parasites, such as overheating or cooling of the aquarium, significant temperature fluctuations in the room where the aquarium is located, high ammonia, nitrite, or nitrate levels, low pH or acidity levels, and so on.

The fish initially become unusually restless and may, for example, jump above the water surface. Then, their coloration changes to a grayish hue, and their bodies become covered in white spots. These spots are cysts that mainly appear on the fins and flanks. As they detach from the fish and fall to the bottom of the aquarium, their spores disperse in the water and contaminate other fish.

As the parasite develops in their bodies, the fish move more slowly or become immobile. They have difficulty breathing, and their scales may detach. When the scales are affected, the death of the fish is likely. Depending on the water temperature, without treatment, mortality will occur in 3 to 4 days for water at 77°F. However, if a fish survives an infection, it will develop resistance to this parasite.

Oodinium or Velvet Disease

Oodinium, also known as “velvet disease,” is caused by the parasite Oodinium pillularis. Like Ichthyophthirius multifiliis, it causes the appearance of white spots, but its treatment differs. It also has a life cycle with distinct stages. The parasite can only be eradicated when it is freely present in the water, outside the bodies of the fish.

Fish Lice

Fish lice, known as Argulus, Livoneca, or Lernaea, are other parasites responsible for diseases. They attach themselves to the fish’s body and can only enter the aquarium through already infected individuals. Therefore, it is crucial to be extremely vigilant when purchasing new fish. Quarantining new individuals after purchase will eliminate the risk of introduction.


Dropsey causes the fish’s belly to swell due to fluid retention caused by organ dysfunction, and their scales become raised. The viral or bacterial origin of the disease is not yet clearly understood. When in doubt, it is preferable to isolate the sick fish.


Lymphocystis is sometimes referred to as “cauliflower disease” because the virus involved forms growths on the fish’s skin. The swollen parts are filled with the virus, which will spread into the water and contaminate the entire aquarium if they burst. Therefore, it is important to treat the sick fish promptly, even if it takes up to 1 year for visible signs of fatigue and weakness to appear.

Non-Contagious Fish Diseases

Columnaris Disease

Columnaris disease is a freshwater bacterial disease that typically occurs in water warmer than 59°F. The skin of infected fish lightens, gray-yellow patches appear, and the gills erode. Mucus also accumulates on the gills, head, and dorsal regions. The disease develops within 24 to 48 hours, and the first fish can die within 48 to 72 hours if no treatment is initiated. In warm temperatures, death can occur within a few hours.

Fin Rot

Fin rot is a disease caused by the bacteria Pseudomonas or Aeromonas. As the name suggests, the condition damages the fins, causing them to fray. The skin may also become dull.


Exophthalmia causes the eye to swell due to fluid accumulation. Acting quickly increases the chances of recovery.

It is essential to monitor your aquarium’s conditions closely. The balance of an aquarium is fragile, and diseases can occur rapidly. Continuous monitoring and regular analysis of water quality, temperature, etc., are recommended.

It is also important to:

    • Not overcrowd the aquarium.
    • Feed the fish correctly (neither too much nor too little) to avoid weakening them and making them more vulnerable to parasites, fungi, viruses, and bacteria.
    • Avoid stressing the fish (which is why children are prohibited from tapping on the glass).
    • Follow the proper procedure for introducing new fish.
    • Maintain effective filtration and good environmental hygiene.
mahatma gandhi portrait

- Mahatma Gandhi

“The greatness of a nation and its moral progress can be judged by the way its animals are treated.”

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