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Common Saltwater Fish Diseases And How To Treat Them

by Brian Dunleavy
Common Saltwater Fish Diseases And How To Treat Them

There is nothing more heartbreaking than losing one of your fish to a disease. In this article, you will learn the common saltwater fish diseases, what to look out for, treatment, and tips on how to prevent them. 

Have you walked up to your aquarium, seen one of your fish sick, and wondered why?

Fish, like other animals, can get sick from diseases. Fish diseases can be one of the most problematic issues in the reefing hobby. 

Now, we are not here to scare you, but instead, to educate you on the common saltwater fish diseases and ways you can prevent them from happening. Let’s get started!

Different Types Of Fish Disease

There are four different categories of fish disease:

  • Bacterial
  • Fungal
  • Parasitic
  • Viral

  • Bacterial

    Bacterial diseases can be any internal, external, or topical infection caused by bacteria. Bacterial infections are most commonly secondary infections that infect fish when they are already feeling run down.


    Fungi are spore-producing organisms that look like white, cotton fluff on your fish. Luckily, fungal infections are less common than the other types.


    Parasites are typically the most common and deadly for fish. They can live inside or on the host (your fish), causing chaos! Parasitic diseases include the dreaded flukes, marine Ich, and velvet disease. 

    Viral (Viruses)

    While not as common as parasitic and bacterial infections, most viral diseases have no treatment. This is because viruses hijack and reprogram cells to make new viruses, making them very dangerous for your fish. 

    Fish First Aid Kit

    To be prepared, we have a first aid kit list for your fish. Just like keeping a medical kit in the cupboard for yourself, it is important to have a well-stocked inventory for your pets too!

    Type Of Fish Disease

    Recommended Medications


    Parasitic (External)

    Parasitic (Internal)




    9 Common Saltwater Fish Diseases 

    The most common saltwater fish diseases are as followed:


    #1 Saltwater Ich (Cryptocaryon irritans)

    Ich, commonly known as white spot disease, is by far the most common and deadliest disease to infect fish and saltwater aquariums. It is caused by the parasite Cryptocaryon irritans. If not controlled, it can quickly transfer to other fish inside your aquarium. 

    As it is often misdiagnosed with velvet disease, it is important to properly identify Ich with the following symptoms to provide necessary treatment. 



    • White spots on the fins and body
    • Scratching/flashing
    • Loss of appetite
    • Rapid breathing



    • Copper-based medication
    • Chloroquine Phosphate
    • UV Sterilizers 
    • Hyposalinity/osmotic shock therapy 


    #2 Marine Velvet (Amyloodinium ocellatum)

    Marine velvet disease is the infection of the dinoflagellate called Amyloodinium ocellatum. It is one to avoid, because of its ability to kill saltwater fish in less than 24 hours. 

    As the dinoflagellate thrives in warm water, marine velvet usually overtakes during the summer months, making it the riskiest time for your saltwater fish. 



    • White spots - too many to count!
    • Rapid breathing
    • Swimming into the powerhead to breathe
    • Sensitivity to light
    • Scratching/flashing
    • Clouding of the eyes, skin, and fins
    • Sudden fish death



    • Copper-based products (Copper Power)
    • Chloroquine Phosphate
    • Rally Pro
    • Quarantine to prevent spreading


    #3 Clownfish Disease (Brooklynella)

    Brooklynella was named clownfish disease because it is most commonly seen in clownfish, but do not be fooled, as it can infect any fish. It is common in wild-caught clowns, but rarely seen in captive-bred clownfish, so better to ask your local fish store (LFS) whether their clownfish are captive-bred or wild-caught before buying. 

    Brooklynella is a kidney bean-shaped microscopic parasite that is deadly to your fish, killing them in a matter of hours to days, so it is best to catch the symptoms early! 



    • Mucus and lesions on the skin
    • Gasping for air
    • Scratching
    • Lethargic behavior
    • Loss of appetite



    • If legal in your state, a Formalin bath is best (45-minute soak), if not use Rally Pro (90-minute soak)
    • After the bath, add Chloroquine Phosphate


    #4 Uronema marinum

    Uronema marinum is a saltwater ciliated protozoa parasite that is often fatal to fish. This fish disease only infects sick fish, so remember to keep your fish healthy and happy at all times to avoid an outbreak. 



    • Ulcer-like wounds that develop into large wounds
    • Mucus formation on the scales
    • Scraping
    • Clamping fins 
    • Often, sudden fish death with no symptoms



    • If your fish has no sores, Formalin or a Rally Pro bath
    • For sores, a freshwater dip
    • Chloroquine Phosphate 
    • Seachem Focus
    • Food treated with Seachem Metro


    #5 Flukes 

    Flukes are the names given for parasitic trematodes or flatworms that live inside your fish’s gills or skin. They are nasty because they feed on the tissue cells and mucus, anchoring down on your fish with their hooked mouths. The flukes have a basic life cycle, reproducing eggs and hatching them into adult larvae. 

    As warm temperatures accelerate their reproductive process, it is important to monitor your aquarium’s water parameters. 



    • Scratching/flashing
    • Twitching
    • Heavy breathing
    • Lethargic behavior
    • Loss of appetite
    • Red spots
    • Missing scales
    • Clamped fins
    • Excess mucus



    • Freshwater dip (5-minutes)
    • Prazipro
    • API General Cure
    • Hyposalinity/osmotic shock therapy for 7 days in a quarantine tank


    #6 Bacterial Infections

    If untreated, bacterial infections will kill your saltwater fish. There are two types of bacterial infections, gram-positive and gram-negative. 

    Gram-positive infections are the most common in marine fish, and unfortunately, they are the most virulent. Often when fish are infected with gram-positive bacteria, the symptoms are extremely mild that they go undetected. 

    Gram-negative infections (as seen in the above photo) are not fun. Many fish that get gram-negative infections die within 24-48 hours after showing symptoms. These strains are known to mess with your fish’s natural immune system, hence the quick fatality.



    • Redness on the body
    • White film or fungus-looking growths
    • Cloudy eyes
    • Fin & tail rot



    • Rally (90-minute dip)
    • API Triple Sulfa
    • Seachem SulfaPlex
    • Seachem NeoPlex
    • Antibiotics for 7-10 days


    #7 Head & Lateral Line Erosion (HILLE)

    Head and lateral line erosion (HILLE), also known as hole-in-the-head (HITH) disease, is common in both freshwater and saltwater fish. HILLE is a chronic condition that creates lesions along the lateral line (the pores that run along both sides of your fish). 

    Although, HILLE is usually non-fatal, the disease does leave behind permanent scarring on your fish’s skin.



    • Discoloration of fish; white or gray fading of the color in patches
    • Superficial erosion on the head and face
    • Large non-bleeding ulcers



    • Removing all activated carbon 
    • Performing large percentage (90%) water changes
    • Increasing nutrition with quality frozen food and vitamins
    • Remove any stray voltage, use a grounding probe


    #8 Internal Infections & Parasites

    Internal infections are caused by worms, parasites, or bacterial infections. They are most common in imported fish, but do not worry, as they are easy to treat if caught early. 

    As internal infections are difficult to detect, it is best to treat your fish with broad medications.  


    • Sunken belly 
    • White stringy poop 
    • Skinny fish unable to gain weight 
    • Loss of appetite
    • Gasping at the surface



    • API General Cure
    • Seachem MetroPlex
    • Seachem Garlic Guard (mixed into the food)
    • Seachem Focus


    #9 Swim Bladder Disease

    Swim bladder disease is an extremely common illness in deep imported saltwater fish, that results in the bladder not functioning properly. This can happen from physical abnormalities, inbreeding, or environmental factors.  

    If untreated, your fish will eventually die.



    • Inability to swim upright and/or downwards
    • Gas bubbles visible in the belly



    • Maintain good water quality.
    • Sodium chloride salt can be added (2-5 grams per liter) - always add with caution if you have fish sensitive to salt
    • You can take your fish to a veterinarian to remove the bubble with a needle/syringe - do not try this at home
    • Antibiotic treatments are applied after surgery to prevent secondary bacterial infections


    How To Prevent Coral Diseases 

    To prevent any of the above coral diseases, you should always:

    • Quarantine new fish
    • Reduce stress
    • Maintain good water quality
    • Provide a healthy diet 
    • Purchase captive-bred fish 



    Unfortunately, saltwater fish are not immune to diseases. Fish diseases like Ich and marine velvet are deadly, so you should never turn a blind eye if your fish appears or acts differently from normal. Preparing yourself with a fish first aid kit and maintaining a healthy environment will help prevent losing any fish. 


    If you have any questions about fish care, maintenance help, or reefing supplies, contact the Reefco team.

    by Brian Dunleavy