Bacterial infection

Many fish diseases are caused by an infection with bacteria. Depending on the type of pathogen, external body parts or organs are attacked.

Definition of the disease / illness including symptoms

If bacteria multiply due to poor husbandry conditions and a damaged immune system, the fish become ill. The scales stand off and fade. Inflamed skin areas develop ulcers with red edges. The fins are reddened and frayed. Via the inflamed fin base the infection spreads to the fish body. Gills, fins and skin show punctiform bleeding. Due to dead tissue some parts of the body are discoloured black. The eyes swell out, googly eyes appear. If the internal organs such as liver and kidneys are also inflamed, fluid accumulates in the abdomen.

The abdomen is distended, the back is pointed. The fish eat less and lose weight. They look tired and retreat to the bottom of the aquarium. The swimming movements are slow and partly jerky. If the gills are very slimy and swollen due to the inflammation, the fish suffer from shortness of breath. They try to gasp for air directly at the water surface.

Summary of the symptoms of a bacterial infection

  • Skin inflammations
  • Bleeding in the skin, muscles and organs
  • the fins are frayed
  • Pop-eyes
  • distended stomach
  • Emaciation
  • reduced feed intake

Diseases caused by bacteria

  • Fin rot and gill rot
  • bacterial ascites
  • Columnaris disease
  • Erythrodermatitis


In the aquarium water, on the body surface and in the intestines of the fish, there are different strains of bacteria. The skin is protected from harmful bacteria by a thin layer of mucus containing antibacterial components. If the protective layer is damaged by injuries or strong temperature fluctuations of the water, a strong reproduction of the bacteria starts.

If new fish are placed in the aquarium, their immune system must first deal with the new bacterial flora. In the first few weeks there is therefore an increased susceptibility to diseases.

Stress, poor quality food and poor hygiene damage the immune system. The fish become more easily ill with bacterial infections. Multi-resistant strains of aeromonads and mycobacteria cause serious diseases.


An infection with multi-resistant bacterial strains always leads quickly to the death of the fish population. The disease can also be fatal for fish that already have injuries. If less lethal bacterial strains are treated with drugs while at the same time optimising the keeping conditions, the inflammation heals within a few days. Severe kidney damage or infections with mycobacteria are always fatal.


In order to strengthen the immune system of the fish, the keeping conditions must be improved. Stress due to overstocking or incorrect species composition should be avoided. Vitamin preparations and high-quality feed support the fish’s defences. Lowering the PH value below 7 reduces bacterial growth. Additional irradiation with a UV lamp kills further bacteria. Cyprinopur has antiviral and antibacterial effects. Mixed into the aquarium water it has a disinfecting effect and accelerates the healing of wounds. Sera Omnipur reliably kills various strains of bacteria. Antibiotics may have to be administered directly in the form of injections.

Sea almond leaves and alder cones have an antibacterial effect. At the end of the treatment, the water must be purified with activated carbon.

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