Saltwater Aquarium Fish Diseases

Marine aquarium

Jessie Sanders

There are a wide variety of diseases to which saltwater fish are susceptible. Because fish essentially live in their toilet and are constantly exposed to various pathogens, a strong immune system is critical to good overall fish health.

Support your fish's immune function by providing an excellent environment, taking into consideration their spatial requirements, desired territory or hiding spots, and preferred tankmates.

Feed a high-quality diet specific to the metabolism and digestion of your fish and be sure to replace it regularly. Test your water chemistry on a consistent basis and make sure maintenance takes top priority.

Eliminating stress from your fishes' environment goes a long way in ensuring their overall health and welfare.

Depending on the disease process, the clinical signs of a sick fish may be fairly obvious or more subtle. Physical signs of disease, such as bumps, fuzzy patches, scale loss, fin rips or changes in color are easy to spot for even novice hobbyists. Behavioral changes, such as changes in appetite, incorrect body positioning, and buoyancy disorders are harder to spot.

The best method of spotting disease in your fish is to check them daily at the same time and be sure to watch every fish closely. Once you know their normal behavior and patterns, it is easier to pick up on any small changes.

  • 01 of 05


    Clown fish resting in anemone

    Muhammed Irwin Day / EyeEm / Getty Images

    Parasites most commonly enter an aquarium when new fish are added without proper quarantine. The stress of transport and adjustment to a new aquarium causes decreased immune function, allowing any parasites riding along to easily replicate and spread.

    Common clinical signs of parasites in saltwater fish include external spots, bruises or scale loss from flashing, increased lethargy and respiratory effort or sudden death.

    Here are the most common saltwater fish parasites:

    Depending on the parasite present, your treatment options may vary. Contact your aquatic veterinarian in order to correctly identify the parasite and prescribe the best treatment available. Many parasite treatments are not coral or invertebrate-safe, so you may need a quarantine tank to treat infested fish.

  • 02 of 05


    Saltwater aquarium

     Jessie Sanders

    There are numerous bacteria species present in your aquarium. A fish's immune system can keep most bacteria in check most of the time. If your fish is stressed due to poor water quality, territory or space disputes, or poor nutrition, their immunity will decrease and they will be more likely to develop a secondary bacterial infection.

    Common signs of bacterial disease in marine fish included lethargy, decreased appetite, ulcers, and sudden death. If you note any of these signs, be sure to test your water quality and evaluate your fish's diet.

    Most of the time, if the primary stressor is identified and eliminated, your fish will not require any additional treatment. Severe infections may require treatment with antibiotics prescribed by your veterinarian.

  • 03 of 05


    Yellow tang in reef
    Yellow Tang (Zebrasomas flavescens)

     Getty Images/Denis Zastanceanu

    There are many viruses that can affect saltwater fish. The most commonly seen is lymphocystis, an iridovirus that also has a freshwater counterpart.

    Clinical signs of lymphocystis include small white or light-colored nodules on the fish's skin and fins. They may also form internally on various organs, which will not be visible.

    The symptoms are easily confused with Marine White Spot disease or benign fin ray fractures. Your veterinarian will take a sample to examine under the microscope to determine the cause of the white nodules.

    There is no effective medication for this disease at present, although your aquatic veterinarian can surgically remove growths from the fin margins. Most of the time, these lesions will resolve on their own, provided your fish is not stressed.

  • 04 of 05

    Gas Bubble Disease

    Goldfish bubbles


    Gas bubble disease occurs when the water becomes supersaturated with pressurized gas and small microbubbles coalesce in the skin and eyes of fishes. The microbubbles are formed in the environment most commonly from misaligned plumbing sucking air into the filter tubing. The dissolved pressurized gas diffuses into the fish through the gills and bubbles out of the blood stream in various places. You may see one large bubble or multiple smaller bubbles in the skin, fins and/or eyes of your fish.

    The bubbles can cause tissue necrosis and lead to fin erosion or internal organ damage. If left untreated, these bubbles can cause irritation and allow pathogens an easy route to enter into your fish's body.

    Please do not attempt to pop the bubbles yourself! Your veterinarian will handle the procedure safely, with your fish properly anesthetized, and provide supportive care. It is critical to locate the source of the microbubbles so the condition does not recur.

    Continue to 5 of 5 below.
  • 05 of 05

    Head and Lateral Line Erosion (Hole in the Head Disease)

    Blue tang with dorsal fin raised in reef
    Blue Tang (Paracanthurus hepatus)

     Getty Images/MirekKijewski

    Head and Lateral Line Erosion, more commonly known as Hole in the Head Disease, does not have a specific definitive cause. It can affect many species and causes erosion of the head and lateral line. Cases may range from a small spot on the head to the entirety of the face and side of the body having skin erosion.

    There are many potential causes of Head and Lateral Line Erosion: poor water quality, high nitrite and nitrate, inadequate nutrition, protozoal parasites. Once the stressor is identified and eliminated, your fish will likely require supportive care in order to make a full recovery. Fish with severe cases may never return to their original appearance.

This is just a small sampling of the diseases that may affect saltwater fish. In order to decrease your fish's chances of getting sick, be sure to quarantine all new fish and invertebrate additions, provide a good environment with enough room for all, good water quality, and feed an appropriate diet. If you are concerned about your fish's health, contact your aquatic veterinarian.