Diseases that Affect Freshwater Aquarium Fish

Freshwater fish tank decorated with live and fake plants

Freshwater fish are susceptible to a variety of illnesses. Fish with reduced immune function or persistent stress from poor water quality, overpopulation, improper diets, and other bad environmental conditions are more susceptible to illness. If these problems aren't addressed, all remedies you try will only be temporary.

For newer fish owners, recognizing sickness in fish might be challenging. Spots, lumps, missing scales, and ragged fins are the most obvious physical indications of ill fish. It takes more skill to recognize more subtle behavioral indicators including inappropriate buoyancy, listing, decreased hunger, or increased breathing effort. If you have any questions regarding a fish, contact your aquatic veterinarian.

  • 01 of 07


    Fish with bruising from parasite infestation

    There are many different parasites that can affect freshwater fish. The most common of these are:

    • Ichthyophthirius multifiilis (White spot disease)
    • Trichodina
    • Mongenean trematodes ("Flukes")
    • Icthyobodo ("Costia")
    • Learnea spp. (Anchor worms)
    • Argulus spp. (Fish lice)
    • Chilodonella

    The majority of parasites enter fish aquariums when new fish are introduced. Fish are commonly seen with a low parasite load at all times. Their immune system prevents them from becoming uncontrollable. When a fish's immune system is compromised by capture, transportation, and introduction into a foreign system, parasites can easily reproduce. With severe infestations, they will jump to all of the fish in your tank and cause flashing, lethargy, decreased appetite, and even death.

    Bruising or missing scales, as well as a loss of appetite, are further symptoms of a parasite infection. Many of these clinical indications, however, might be confused with those of other disorders, making diagnosis difficult.

    Treatment options vary depending on the parasite involved. Most will be temperature-dependent. Your aquatic veterinarian will prescribe the correct treatment following a microscopic diagnosis.

  • 02 of 07



    Most bacterial infections in fish are secondary to other primary stressors. Typical causes of chronic stress include poor water quality, overcrowding, and inappropriate diet.

    Aeromonas, Vibrio, Edwardsiella, Pseudomonas, and Flavobacterium spp. cause the most prevalent aquatic bacterial illnesses. Infections caused by Mycobacterium spp., generally known as fish TB, can also infect immune-compromised people. Mycobacterium spp. has no treatment, and system maintenance needs consultation with an aquatic veterinarian.

    Edema of the skin (dropsy), ulceration, decreased appetite, fin erosion, lethargy, subsequent fungal or parasite infections, and rapid mortality are all symptoms of a bacterial illness.

    Flavobacterium spp., sometimes known as "cotton wool" sickness, is frequently misdiagnosed as a fungus.

    Excessive use of over-the-counter "antibiotics" has resulted in antibiotic-resistant microorganisms that require highly harsh treatment. These chemicals also destroy your biological filtering system, causing New Tank Syndrome, which puts your fish at risk chevalier.

    Consult an aquatic veterinarian before dumping in a bunch of items. They can prescribe a suitable and successful treatment. They could suggest a bacterial culture and sensitivity testing to figure out which bacteria is causing the problem and which antibiotic is best for treating it.

  • 03 of 07

    Fungal Diseases


    Fungal illnesses are generally exclusively detected on the skin of deceased fish. These patches are easily formed on dead scales, rotting fins, or traumatic injury sites. They're also present on fish with weakened immune systems as a result of a viral infection or heredity.

    Most fungal infections resolve on their own once the dead skin is removed. Persistent infections may require aggressive therapy coordinated by your veterinarian.

  • 04 of 07



    There are a few viral diseases in freshwater fish that can affect common aquarium species. The most common is lymphocystis.

    An iridovirus causes little white pimples that might be mistaken for fin ray fractures or White spot illness. To identify between the many causes of white spots on your fish, your veterinarian will need to inspect it.

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  • 05 of 07


    Very tiny microbubbles in your tank frequently cause gas bubble illness. A misplaced piece of piping or abrupt temperature swings, such as during a water change, are typical causes.

    Skin bubbles, fins, and eyeballs are all clinical indicators. The gills may have tiny alterations that your veterinarian may inspect under a microscope. To cure fish with gas bubble illness, you'll need veterinarian help.

  • 06 of 07



    Fish, unfortunately, are not immune to cancer. There are several forms of bacteria that can infect your freshwater fish, and treating them as soon as possible is vital to their health.

    Most cancers present as abnormal growths on the outside or inside of the fish. You may also see a decrease in appetite, increased lethargy or inability to reproduce.

    Contact your aquatic veterinarian right away if you believe your fish has cancer. Some can be surgically removed if detected early enough, dramatically enhancing the quality and length of life of your fish.

  • 07 of 07

    Polycystic Kidney Disease


    Goldfish, one of the most common freshwater aquarium fish, are susceptible to this illness. The etiology of this condition, in which cysts build in the fish's kidneys and prevent them from operating properly, is unclear. Your fish's body begins to fill with water, and they resemble a water balloon. Other than palliative care, there is no cure for this condition, which is sadly fatal.

When it comes to treating any disease in freshwater fish, early identification and successful treatment are crucial. Always ensure that your fish are in the right habitat, with clean water and a balanced food. Remember that veterinarian aid will likely result in safer and more successful results than any DIY fish treatments.