Freshwater Aquarium Fish Diseases

Freshwater fish tank decorated with live and fake plants

Getty Images/mtreasure 

There are many diseases that can affect freshwater fish. Fish are at higher risk of disease when they have compromised immune function or chronic stress from poor water quality, overcrowding, inappropriate diets, and other poor environmental concerns. These issues need to be addressed or any treatments attempted will only have a temporary effect.

Recognizing disease in fish can be difficult for newer fish owners. Obvious physical signs, such as spots, lumps, missing scales or frayed fins are the easiest signs of sick fish. More subtle behavioral signs, such as incorrect buoyancy, listing, decreased appetite or increased respiratory effort take more experience to spot. If you are concerned about a fish, call your aquatic veterinarian for more assistance.

  • 01 of 07


    Fish with bruising from parasite infestation

    Dr. Jessie Sanders

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

    Most parasites enter fish systems when new fish are introduced to the aquarium. Commonly fish exist with a low level of parasites on them at all times. Their immune system keeps them from getting out of control. When a fish is stressed out from capture, transport and introduction into a foreign system, their immune system is weakened and the parasites can replicate easily. They will then jump to all the fish in your tank and cause flashing, lethargy, decreased appetite, and even death with severe infestations.

    Other clinical signs of a parasite infestation include bruising or missing scales and decreased appetite. Many of these clinical signs can present with other diseases, though, so diagnosis can be tricky.

    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


    Koi with side ulcer from persistent bacterial infection

    Jessie Sanders

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

    The most common aquatic bacterial infections are caused by Aeromonas, Vibrio, Edwardsiella, Pseudomonas and Flavobacterium spp. There are also Mycobacterium spp. infections, commonly known as fish tuberculosis, which can potentially infect immune-compromised humans. There is no cure for Mycobacterium spp. and it requires consultation with an aquatic veterinarian for system maintenance.

    Clinical signs of a bacterial infection include edema of the skin (dropsy), ulceration, decreased appetite, fin erosion, lethargy, secondary fungal or parasitic infections and sudden death.

    Flavobacterium spp., also known as "cotton wool" disease, is commonly mistaken for a fungus when it is actually a bacteria.

    Excessive medication with over-the-counter "antibiotics" has lead to resistant bacterial strains that require very aggressive treatment. These products also wipe out your biological filtration and lead to New Tank Syndrome, which can compromise your fish even more.

    Before you just dump in a bunch of stuff, consult with an aquatic veterinarian who can recommend an appropriate and effective treatment. They may recommend a bacterial culture and sensitivity testing in order to identify the bacteria causing the problem and the best antibiotic to treat it.

  • 03 of 07

    Fungal Diseases

    Koi with secondary fungal growth on sides and fins

    Dr. Jessie Sanders

    Fungal diseases are usually found only on dead fish skin. These patches form readily on dead scales, decaying fins, or sites of traumatic injury. They are also found on fish with compromised immune systems due to concurrent viral disease or genetics

    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


    Cichlid tank decorated with live and fake plants

    Getty Images/mtreasure 

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

    This is caused by an iridovirus and presents as small white bumps that can be confused with fin ray fractures and White spot disease. Your veterinarian will need to examine your fish in order to distinguish between the various causes of white spots on your fish.

    Continue to 5 of 7 below.
  • 05 of 07

    Gas Bubble Disease

    Blood parrot cichlid

    Getty Images/DigiPub

    Gas bubble disease is usually caused by very fine microbubbles in your tank. This is commonly caused by a misaligned piece of plumbing or rapid changes in temperature, such as during a water change.

    Clinical signs include bubbles in the skin, fins and eyes. There may be microscopic changes in the gills, which can be examined by your veterinarian under a microscope. You will require veterinary assistance to treat fish with gas bubble disease.

  • 06 of 07


    Koi with skin cancer aka hikui

    Dr. Jessie Sanders

    Unfortunately, fish are not immune to various cancers. There are many types that can potentially infect your freshwater fish and it is critical to their health that they are treated as early as possible.

    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.

    If you suspect your fish has cancer, contact your aquatic veterinary ASAP. If caught early enough, some tumors can be surgically removed, greatly improving your fish's quality and length of life.

  • 07 of 07

    Polycystic Kidney Disease

    Deceased goldfish with polycystic kidney disease

    Dr. Jessie Sanders

    This disease is found in goldfish, one of the most common freshwater aquarium fish. The cause of this disease is unknown, wherein cysts form in the kidneys of the fish, preventing them from working correctly. Your fish's body starts to take on excess water and they start to resemble a water balloon. There is no treatment, other than palliative care, and the disease is unfortunately terminal.

As with any animal, early detection and effective treatment are key when treating any disease in freshwater fish. Always make sure your fish are in the appropriate environment with good water quality and an adequate diet. Before you attempt any DIY fish treatments, remember that veterinary assistance will likely result in safer and more effective results.