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The Beginner Guide To Saltwater Fish Disease

Table of Contents

Disclaimer: I am NOT an expert.  I am NOT a veterinarian.  I am NOT a medical professional.  But I have spent years trying my best to compile the most accurate information available.  Saltwater hobbyist rarely agree on anything, especially when it comes to fish disease diagnosis and treatment.  Please use this guide as a starting point for your research.

Saltwater fish disease is one of the most complex topics in this hobby, and can be extremely overwhelming.  There are so many blogs and online communities that discuss every detail of this topic.  But I haven’t found one to put it all together into an easy to digest yet comprehensive guide.  That’s what this bad boy is for!

This is a really, really hard topic, and here’s why:

  • No matter what you do, your fish will often die anyway.
  • By the time you notice a problem with your fish, it is often too late.
  • Some species respond well to a certain med while others don’t.
  • Meds can kill your beneficial bacteria and inverts.
  • Until you have experience, you’ll have no baseline to know what is normal and what is not.
  • Many diseases present with similar symptoms, but require different treatments.

You get the idea.

Hands down the two best things you can do is try to prevent disease from happening and use a quarantine tank.

Enjoy the guide, and email me with any suggestions or inaccuracies.

For a comprehensive guide to fish disease, check out the book below by Edward Noga.  It is considered the fish disease bible.

I) Why Saltwater Fish Get Sick

1. Stress

Just like in humans, stress takes a toll on the body and lowers the immune system.  Fish are no exception.  Different diseases will always be present in your water column, but a strong fish will be able to fight them off and remain healthy.

2. Aggressive Tank Mates

All of the options in The Beginner’s Guide To Saltwater Fish are community oriented, and can play well with others (with a few caveats of course!). Aggressive fish can harass and nip at the fins of others, causing tears, abrasions, and overall stress.  This can lead to a weakened immune system and disease.

3. Water Parameters

Unhealthy water parameters such as high ammonia and nitrate levels increase the chance of fish disease.  Keeping your tank too hot/cold or maintaining hyper/hypo salinity are also factors.

But even worse than all of that in instability.  Fish are quite good at adapting to their environment (usually), but constant changes can stress them out.

4. Injury

Abrasions and injuries leave a fish much more vulnerable to a secondary bacterial, viral, or fungal infection.

5. Tank Size

Every species of fish has different tank requirements for them to thrive.  For example, putting more than two clownfish in a 20-gallon tank will likely cause aggression issues.  Of trying to keep a pufferfish in a 40-gallon will likely not end well. Make sure your tank is the correct size for your livestock.

6. Overstocking

There is no golden rule to how many fish you can have in a tank.  But there can become a point where you have too many fish.  This can lead to aggression issues, as well as the inability to maintain a healthy biological filter.

7. Age

Just like our species, fish are more prone to disease as they reach the end of their life.

8. There Is Always Disease In A Tank

While it is possible to keep certain diseases and parasites out of your tank using proper quarantine measures, you can’t keep everything out.  Various bacteria will always exist in your tank, so a stressed or injured fish will be more susceptible to disease.

II) How To Prevent Fish Disease

Use a quarantine tank.  I know, I know, must hobbyists don’t. But don’t be like most hobbyists!  A QT is not some magic cure all, but it allows you time to monitor your fish for possible problems, and treat them away from your display tank

2. Dips

There are both medicated dips and freshwater dips.

If you are not going to quarantine, you can at least use this two-part Blue Life Safety Stop.  It’s not a cure-all, but it can help remove parasites, bacterial, and fungal infections. 

3. Keep Stable Water Parameters

You can accomplish this by testing frequently, cleaning your filters regularly, doing weekly or biweekly water changes, and using a temperature controller such as this Bayite.

4. Remove Aggressive Fish

While it is common for your fish to harass any new tank members, if this does not go away in a day or so, you may need to remove the aggressor.

Aggressivity can also spring up at any time.  For example, I had two clownfish in my 20-gallon office tank for months, and then over the course of a week, the female started bullying the male relentlessly. So she was out of there and into the 120-gallon!

5. Drip Acclimate All New Fish

For detailed instruction on drip acclimation, check out The Beginners Guide To Saltwater Fish.

Taking your time, floating your bags to equalize the temperature, and then drip acclimating to equalize water parameters will reduce fish stress and help prevent an outbreak of disease.

6. Don't Overstock

Pretty self explanatory here.  It’s best to err on having too few fish rather than too many.

7. Add Fish Slowly

What I mean is don’t go out and buy all of your fish at once and throw them in your tank.  Take your time.  Adding too many fish at once will mess with your water chemistry, and your biological filter may not be able to keep up.

So if your goal is to have ten fish, start with a couple, and add from there.

8. Buy Community Fish

Fish are usually grouped into three categories based on temperament: peaceful, semi-aggressive, and aggressive. If you are a beginner, don’t buy any aggressive fish.

Check out The Beginner’s Guide To Saltwater Fish for our top ten choices.

III) Signs & Symptoms Of A Sick Fish

This can be really hard because until you have experience with each species of saltwater fish, you won’t have a baseline for normal. So check out the online forums or go to your LFS to get your questions answered.

All of the signs and symptoms below can be indicators that your fish is sick, but must be taken in context to the species and your specific situation.  For example, a fish that doesn’t eat may not be sick… it might just not like that type of food!

1. Loss of Appetite

Some fish are grazers and eat slowly over the day, while others are opportunistic and attack all new food.  It’s important to know the eating habits of each of your fish.

Regardless, if you notice a change in the eating habits of your fish, something could be wrong.

2. Flashing

Flashing is when fish quickly rub themselves on your rock.  This can often mean they are trying to get off some sort of infection or parasite. This can also lead to abrasions and secondary infections.

3. Bulging Eye

Literally looks like the eye of the fish is bulging out of its body. Much more common in the freshwater hobby.

4. White Dots

White dots on the skin, usually the size of a grain of sand, are often a sign of Marine Ich or Velvet, both of which are bad.

5. Red Gills

Often the sign of a bacterial or parasitic infection. Often accompanied with rapid breathing.

6. Rapid Breathing

Can be hard to diagnose unless you know the baseline for each of your fish.

7. Hiding

Some species always hide, but others are meant to be swimming freely in your water column. Watch for changes in behavior.

8. Cotton Looking White Growths

Often the sign of a fungal or viral infection.

9. Cloudy Eyes

Could be an internal bacterial infection.

10. Frayed/Disintegrated Fins

Either a sign of fish aggression or disease.

11. Missing Scales Or Red Growths

Again, either a sign of fish aggression or disease.

12. Lethargy

Important to know the baseline for each of your fish to diagnose this symptom appropriately.  Some fish are just naturally lethargic!

IV) Four Types Of Fish Disease

The most common fish disease and their treatments will be discussed in detail in just below in section V) of this guide. There are four different categories of fish disease:

1. Bacterial

Can be internal, external, or topical, any infection caused by bacteria.  Can be Gram-positive or Gram-negative (huh? keep scrolling to section V for a detailed explanation!).  For a full explanation and list of antibiotics, check out The Beginner Guide To Saltwater Fish Medication & Treatment.  Bacterial infections are often secondary infections that creep in after the fish is already sick.

2. Fungal

Not a plant or an animal, fungi are spore-producing organisms such as mold, yeast, and mushrooms. The most common saltwater aquarium fungus usually presents as a white cotton-like fuzzy patch on fish. Not super common in this hobby.

3. Parasitic

Probably the most common and often the most deadly, a parasite is simply any organism that lives in/on another organism and gets its food at the other’s expense. Marine Ich, Velvet, and Flukes are prime examples in the saltwater hobby.

4. Viral

Not as common as parasites or bacterial infections, there is also no treatment for viruses.  While bacteria are single-celled creatures, a virus cannot survive without a host, and they are tiny when compared to bacteria. Viruses work by hijacking and reprogramming cells to make new viruses.

V) Common Saltwater Fish Diseases & Treatments

Disclaimer: Let me repeat one more time…I am NOT an expert.  I am NOT a veterinarian.  I am NOT a medical professional.  But I have spent years trying my best to compile the most accurate information available.  Saltwater hobbyist rarely agree on anything, especially when it comes to fish disease diagnosis and treatment.  Please use this guide as a starting point for your research.

Parasitic Diseases

© Marine Depot
      • Common Name(s): White Spot Disease, Saltwater Ick
      •  Scientific Name: Cryptocaryon Irriatans
      • Description: One of the most common saltwater fish diseases.  Often confused with Marine Velvet (which is much, much deadlier!).  Marine Ich penetrates the skin and gills.  Most healthy fish are able to fight off this parasite. Often presents with small salt sized white spots, but can primarily infect the gills with no external signs. It is difficult to remove from your tank due to its lengthy life cycle. 
      • Symptoms: White spots on fins, gills, and body.
      • Treatments: Seachem Cupramine is the most common and effective treatment.  The Tank Transfer Method is also highly effective.  A freshwater dip can temporarily eradicate external parasites, offering some relief, but it is not a cure. A UV sterilizer can help kill the free floating phase of the parasite.  Some hobbyists have reported success using a combination of Ruby Reef Rally and Ruby Reef Kick Ick.
      • Our Recommendation: Either a quarantine tank with Seachem Cupramine or the Tank Transfer Method.

2) Marine Velvet

By Mydigitalife - Own work, CC0,
      • Common Name(s): Marine Velvet, Gold Dust Disease
      •  Scientific Name: Amyloodiunium Ocellatum
      • Description: Has a similar life cycle to marine ich, but is much, much more deadly. Often presents with the same symptoms as ich, so can be extremely difficult to decipher between the two parasitic infestations.
      • Symptoms: Smaller round white dots in comparison to ich.  Covers the entire fish. Marine ich is larger and oval shaped by comparison.
      • Treatments: Seachem Cupramine and a freshwater dip… that’s it.  Chances are, if you have diagnosed it as marine velvet, it is too late and your fish will die.
      • Our Recommendation: Seachem Cupramine is likely the only chance you have (only in qt), and a freshwater dip can temporarily relieve symptoms.

3) Flukes

© Humblefish, December 4th, 2015.
      • Common Name(s): Gill Flukes, Skin Flukes, Marine Flukes
      •  Scientific Name: 13 families, but three common to saltwater: Ancyrocephaladae, Gyrodactylidae, & Capsalidae.
      • Description: Parasitic flatworms, flukes feed on tissue of fish exterior and gills.  Small and transparent, they are difficult to spot.  If placed in a freshwater dip, flukes will fall off and turn opaque. Flukes can lay eggs rapidly, and the new free swimming babies will find a new host.
      • Symptoms: Rapid breathing, frayed fins, loss of appetite, twitching head, discolored blotches on fish, flashing, white translucent spots that look similar to ich.
      • Treatments: A freshwater dip to to kill external flukes, followed by an antiparasitic.
      • Our Recommendation: If the infestation is bad, do a freshwater dip first to kill the external worms and give relief to your fish.  Then follow up with Hikari PraziPro.

4) Clownfish Disease

© Humblefish, May 8th, 2016.
      • Common Name(s): Anemonefish Disease, Brook
      •  Scientific Name: Brooklynella Hostilis
      • Description: Clownfish are notoriously susceptible to brooklynella, but other fish can host it as as well.  Presents similarly to marine velvet. Will often attack the gills first. Its lifecycle is completely contained in/on the host, so there is no drop-off cyst stage. It can kill fast, so a quick intervention is crucial.
      • Symptoms: Rapid breathing, flashing, loss of appetite, lethargy. Advanced stages will look like the skin is sloughing off. Presents similar to marine velvet but with more slime production.
      • Treatments: Formaldehyde, sold commonly under the name formalin.
      • Our Recommendation: Mardel Quick Cure & Ruby Reef Rally both contain formalin. We have personally used both and had success, but we had to dose for several weeks, not several days. Choose one and follow the directions closely.

5) Black Spot Disease

© Humblefish, August 4th, 2016.
      • Common Name(s): Black Ich, Tang Disease.
      •  Scientific Name: Turbellarian flatworms
      • Description: Looks like small black dots covering the skin.  These are actually flatworms. Much less dangerous when compared to marine ich / marine velvet. Similar life cycle to ich & velvet, with an active infestation stage, a fall off into substrate and reproduce stage, and finally a free-swimming looking for a host stage.
      • Symptoms: Black spots about the size of a grain of salt, flashing, lethargy, loss of appetite.
      • Treatments: Praziquantel, hyposalinity, & formalin. Freshwater dips can provide temporary relief as well.
      • Our Recommendation: Hikari PraziPro would be our go-to option, with a freshwater dip if the infestation is bad.

6) Uronema

© Humblefish, May 8th, 2016.
      • Common Name(s): Uronema
      •  Scientific Name: Uronema Marinum
      • Description: Most often found on Chromis, although can appear on any fish. Uronema is tricky once it’s identified because it doesn’t need a host to survive.  So if it is in your tank, the only way to truly be sure you are rid of it is to break down and sterilize your tank.
      • Symptoms: Red sores.
      • Treatments: Formalin followed up by antibiotics for a couple weeks to stave off a secondary infection.
      • Our Recommendation: Use either Mardel Quick Cure or Ruby Reef Rally. Once uronema is gone, follow up with a two week course of Seachem Metroplex and Seachem Focus to help prevent a secondary infection.

Bacterial Infections

There are so many types of bacterial infections, but they at least present with similar symptoms. Often a secondary infection after a parasitic outbreak. Infections can be both internal and/or external.

The problem with bacterial infections is proper identification. Different types of bacteria respond to different types of antibiotics. 

      • Cause: Usually follows a parasitic infection or is an issue with poor water quality.
      • Symptoms: Frayed or disintegrated fins (fin/tail rot), open sores, abdominal swelling, extruded eye(s) (popeye), cloudy eye(s), white cotton-like growths around mouth, fins, or body.
      • Treatments: A UV sterilizer can help eliminate the free floating bacteria, and antibiotics will take care of the bacteria.
      • Our Recommendation: Use our Beginner Guide To Saltwater Fish Medication & Treatment to choose the correct antibiotic based on disease symptoms.

Fungal Infections

These are relatively rare in the saltwater aquarium hobby, but can appear if a fish is stressed.  Just like bacteria, fungus is probably always present in your aquarium, but a healthy fish is able to fend it off.

© My First Fish Tank, 2019

Viral Infections

The most common type of virus in saltwater fish is called lymphocystis.  It appears as a white cauliflower looking growth (or white pebbles, almost wart-like). There is no known antiviral treatments, so you just need to keep your water clean and your fish free from stress. Can be harmless and usually resolves on its own.

© Chris Grondahl,
      • Name: Lymphocystis.
      • Cause: Poor water quality and fish stress.
      • Symptoms: White cauliflower looking growths, white pebbles on outside of fish body. Can be wart-like in shape.
      • Treatments: No known antiviral treatments. 
      • Our Recommendation: Improve your water quality through a large water change (25-50%), test your water parameters and improve them wherever possible. Remove any aggressive fish or other stressors from your tank. Lymphocytes are usually harmless and will resolve on their own.