Using a Saltwater Aquarium Fish Diagnostic Tool

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When one or more of the fish in your saltwater aquarium first start acting or looking "weird," it is the time to start doing the research to determine what the problem is. Most marine fish diseases or afflictions, if detected, diagnosed and treated in their early stages can be corrected and cured.

To help with the diagnosis, check this list of potential symptoms. It can help you focus on a diagnosis and start early treatment. Utilizing a quarantine tank for treatment is usually the best course of action. If a number of your tank's occupants are showing the same symptoms, the entire tank should be treated.

Keep in mind that many of the treatments for fish diseases will weaken your tank's biological filter, so steps should be taken to minimize their effect on the "good bacteria" in your system.

  • 01 of 11

    No Appetite

    It has been said that when something goes wrong with a marine fish, the first thing to go is their appetite. Lack of appetite in marine fish is normally the first symptom of a more serious problem. When a fish is first introduced to a new aquarium system it ​is not unusual for a fish not to eat for a day or two because of their new environment and/or shipping stress, but they should start eating after that. Use tips on getting new fish to eat In your marine aquarium to help your picky eaters start eating.

  • 02 of 11

    White Dots on a Fish's Skin or Fins

    This is a sign of marine parasites such as Cryptocaryon (marine white spot disease), Oodinium (marine velvet, coral reef fish disease), Brooklynella (Brooklynellosis or anemonefish disease) which can be difficult to identify. Since the treatment for these diseases is different, it is important to make a correct identification of the parasite before selecting a treatment.

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    Popeye (an eye which is enlarged and protruding from its socket) is the result of an infection in the eye itself. It is normally caused by a scratch, abrasion or another injury to the eye which has become infected with bacteria in the surrounding water.​

  • 04 of 11

    Cloudy Eyes

    Cloudy eyes may be a sign of a bacterial infection due to an injury on another part of the fish. The bacteria enters the body of the fish and then can spread to the eyes, causing the cloudy appearance.

    Introduction of the bacteria into the aquarium system can be from poor environmental conditions. If conditions are poor enough, the bacteria can bloom and overrun even the healthiest of fish. White cloudy water with the presence of sores on the fish can be signs of this problem.

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  • 05 of 11

    Frayed Fins

    Frayed fins on a fish are frequently caused by ammonia burns which have literally "burnt" the outer edges of the fins. The normal treatment for this is the same as for other infections.

    Frayed fins can also be due to a bacterial infection caused by Flavobacterium columnare. This bacterium can cause a fuzzy appearance to the skin or fins, similar to a fungal infection.

  • 06 of 11

    Rapid Gilling

    Rapid gilling (increased movement of the operculum by breathing rapidly) is a sign that the fish is not getting enough oxygen into its system. This can be caused by excitement, exercise or by the fish's gills being clogged by mucus, which inhibits the gills from being able to perform gas exchange (suffocation). If the fish is just swimming along normally, or at rest and it is gilling rapidly, it is a sign that the gills are damaged from excess mucus from bacteria, parasites, or chemicals in the water such as ammonia or chlorine.

    Parasites lodged in the gills cause irritation to the point that the fish's immune system will produce more mucus to protect the gill membrane. Treat the fish for the appropriate parasite as soon as possible.

    The other frequent cause of rapid gilling is an over-abundance of ammonia in the packing (shipping) water or aquarium water. While fish and invertebrates can tolerate a certain (low) level of ammonia (a toxin), higher levels can "burn" (irritate) the gills causing them to produce more mucus as a defense, which inhibits their ability to take in oxygen.

    High levels of ammonia can also cause other problems with saltwater fish such as ragged, frayed or burnt fins and/or bacterial infections.

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    Open Sores

    Open sores (raw spots) on a fish are frequently the result of a parasitic infestation that has left openings in the skin that have become infected by bacteria. The treatment is the same for any bacterial infection.

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    Red fins

    Red fins on a fish usually indicate an internal bacterial infection caused by ammonia burns, skin parasites or trauma that has allowed bacteria to enter the body and spread.

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  • 09 of 11

    Faded Colors

    Faded colors on a fish can be due to something as simple and harmless as the fish assuming its nighttime colors or be a sign of a more serious condition. Faded colors are usually experienced in conjunction with other symptoms when they are not the fish's nighttime colors.

  • 10 of 11

    Bloating or Abdominal Swelling

    Abdominal swelling or bloating is usually a sign of a kidney infection that prevents normal elimination of fluid from the body, often a common problem with ​wrasses that burrow in the substrate. When fish are collected in the wild from deep water and brought to the surface, swelling or bloating can be caused by the "bends" (lack of decompression), which is the result of bringing a fish up from deeper waters too quickly.

  • 11 of 11

    Scratching on Rocks

     When a fish is seen scratching or rubbing on rocks or other objects (even the substrate), it is a sign that the fish's skin is irritated and the fish is trying to remove the irritating material. This is often called "flashing" due to the light color of the fish’s belly becoming visible when they rub their sides on the rocks. The causative organism is normally a protozoa, either Cryptocaryon (marine white spot disease) or Amyloodinia (Oodinium, marine velvet, or coral reef fish disease).

Article Sources
The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Got A Sick Fish?American Veterinary Medical Association, 2020

  2. Fish disease and human health. Government of Western Australia, 2020

  3. Do Fish Get Sick?. Aquatic Veterinary Services, 2020

  4. Signs Of Illness In Fish. Arizona Exotic Animal Hospital, 2020