Dropsy/Bloat in Aquarium Fish, Diagnosis and Treatment

Dropsy or Bloat is an old medical term that was once used to describe swelling due to an accumulation of fluids in the tissues or body cavities, such as the abdomen. Fish suffering from Dropsy often have a hugely swollen belly, hence the origination of the disease name.

The disease is actually an infection caused by bacteria that are commonly present in all aquariums. Consequently, any species of fish can be stricken with Dropsy, but healthy fish rarely fall prey to the disease.

Fish are only susceptible when their immune system has been compromised by some stress factor. If all the fish in the tank are under stress, it’s quite common for the entire tank to become infected. It is also possible for only one or two fish to fall ill, especially when prompt action is taken to prevent the spread of the disease.

As the infection progresses, skin lesions may appear, the belly fills with fluids and becomes swollen, internal organs are damaged, and ultimately the fish will die. Even with prompt treatment, the mortality rate is high. Only fish that are diagnosed in the early stages of the infection are likely to respond to treatment.


  • Names: Bloat, Dropsy
  • Disease Type: Bacterial (gram-negative organism)
  • Cause / Organism: Multiple causes


Symptoms vary widely. Some fish will have the classic swollen belly, others display skin lesions, while still others show few symptoms at all.

This variability is what makes diagnosis difficult. In most cases, a number of symptoms are observed, both physical and behavioral. Those include:

  • Grossly swollen belly
  • Scales stand out (pinecone appearance)
  • Eyes bulge
  • Gills become pale
  • Anus becomes red and swollen
  • Feces pale and stringy
  • Ulcers form on the body along the lateral line
  • Spine may become curved
  • Fish clamps fins
  • Fish becomes lethargic
  • Fish stop eating
  • Fish hangs near the surface

These symptoms are the result of the progression of the infection. Internal organs are affected, most notably the liver and kidneys. Anemia occurs, causing the gills to lose their normal red color. As the abdomen fills with fluid, organs are pushed aside, sometimes causing the spine to curve. Scales protrude from the body, giving the appearance of a pine cone. This symptom is a classic indication of a severe infection.

The bacterial agent that causes Dropsy is one of the several gram-negative bacteria commonly present in aquarium habitats. The underlying cause of fish becoming infected in the first place is a compromised immune system that leaves the fish susceptible to infection. This can happen as the result of stress from a number of factors, such as the following:

  • Poor water quality
  • Ammonia or nitrite spikes
  • Large drop in water temperature
  • Stress from transportation
  • Improper nutrition
  • Aggressive tankmates
  • Other diseases

Generally a single or short-term exposure to stress will not compromise the ability of the fish to fight infection. In most cases of lowered immunity, the stress exposure has happened for an extended period of time, or several stress factors have occurred in rapid succession.


Dropsy is not easily cured. Some recommend that all affected fish be euthanized to prevent the spread of the infection to healthy fish. However, if detected early it is possible to save affected fish. Treatment is geared towards correcting the underlying problem and providing supportive care to the sick fish.

It is important to move the infected fish to another tank to separate them from the healthy fish. Meanwhile, perform a water change on the original tank and monitor the fish closely for appearance of symptoms. Salt should be added to the hospital tank, at the ratio of one teaspoon per gallon of water. Keep the tank scrupulously clean, and perform weekly partial water changes.

Provide a variety of fresh, high-quality food. Often this is enough to resolve the infection in cases that are not advanced. Keep the fish under observation for several weeks after symptoms disappear.

An effective treatment is to add an antibiotic to the food. With flake food, use about 1% of antibiotic and carefully mix it in. If you keep the fish hungry they should eagerly eat the mixture before the antibiotic dissipates. Antibiotics usually come in 250 mg capsules. If added to 25 grams of flake food, one capsule should be enough to treat dozens of fish. A good antibiotic is chloromycetin (chloramphenicol). Or use tetracycline. If you feed your fish frozen foods or chopped foods, try to use the same ratio with mixing. As a last resort add at most 10 mg per liter of water. Also, if unkempt conditions are the suspected cause, correct it.


As in many diseases, prevention is the best cure. Almost all the factors that stress fish enough to cause them to be susceptible to infection can be prevented; because poor water quality is the most common root cause of stress, tank maintenance is critical. Other factors to keep in mind include:

  • Perform regular water changes
  • Keep the tank clean
  • Clean the filter regularly
  • Avoid overcrowding the tank
  • Do not overfeed
  • Use flake foods within one month of opening
  • Vary the diet

If the tank is well maintained, and the fish fed a healthy diet, outbreaks of Dropsy are unlikely.


For more info on Fish diseases and their treatment, see Aquarium Fish Diseases and Treatments.

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