Common Saltwater Fish Diseases – Symptoms And Treatments

Updated: Dec 8/2020

This guide will cover some of the most common saltwater fish diseases including symptoms and treatment.

Unfortunately, no matter how well you take care of your saltwater fish, there is always a chance they will become sick.

Although you may not be able to completely protect them against certain illnesses, there is a lot you can do to prevent many of the most common ones.

It’s important that you learn about the different types of diseases, what signs to look for, and what treatments are available.

One of the most frustrating aspects of fishkeeping is that they can become ill with little to no warning.

Saltwater fish can become sick very quickly, and some diseases can be fatal if they are not immediately treated.

4 Common Saltwater Fish Diseases

1. Marine Velvet

2. White Spot Disease

3. Tuberculosis

4. Fin Rot

Marine Velvet

Also referred to as velvet, this is among the most common saltwater fish diseases aquarium fish suffer with.

Marine velvet can quickly spread to all of the fish in the tank if it is not treated quickly.

Marine velvet is caused by a one-celled organism or a dinoflagellate. The scientific name of the organism is Amylooddinium Ocellatum.

It occurs naturally in most saltwater aquariums, and for the most part, it functions as a parasite.

It is very difficult to control once its population begins to increase in a saltwater tank, and this is why the disease seemingly appears overnight.

Once a saltwater fish becomes infected with the parasite, there is a range of symptoms that will begin to appear.

The fish may begin to bleed around the gills, or show signs of fatigue or stress.

The one sign that is more difficult to detect is the loss of lung tissue. As the disease progresses, it affects the gills and lung tissue.

This causes the fish to lose the ability to move oxygen through the membranes in the gills and the fish will suffocate.

Because this disease has such a high mortality rate, immediate action must be taken at the first sign of distress or inflammation in the fish.

Copper is a treatment that counteracts this disease, but it must be introduced into the aquarium at the right levels because highly concentrated copper is toxic to saltwater fish.

Always follow the proper dosing recommendations.

Marine White Spot Disease

This disease is commonly referred to as Marine Ich. It is also caused by a parasite called Cryptocaryon irritans.

The tell-tale sign of this disease is the appearance of white spots that covers the gills, fins, and the body of saltwater fish.

Marine White Spot disease is similar to freshwater Ich. The only difference is that it is caused by a different organism.

Both of these diseases are extremely contagious. The disease can be more aggressive in saltwater fish who already have injuries or are under stress.

In addition to the white spots, some of the other noticeable symptoms are:

  • Flashing
  • Fatigue
  • Intense mucus production
  • Ragged fins
  • Pale gills
  • Cloudy eyes

Even though one of the treatment regimens for freshwater Ich is increasing the water temperature in the tank, this does not work as well for saltwater fish.

It is recommended that you remove the infected fish from the main tank and place them into a quarantine tank.

You can try a copper treatment which is to be added to the water at a rate of 0.15 to 0.25 mg per liter. A great product to use is Cupramine by Seachem.

You can also try to kill the parasite by using a hyposalinity treatment.

This is done by bringing the saltwater level down 10% by diluting saltwater levels at a small percentage in your quarantine tank.

saltwater quarantine tanks
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This disease affects saltwater fish more than freshwater fish. Nicknamed the ‘wasting disease’, tuberculosis often presents itself through fin erosion.

Fish may also exhibit other signs such as:

  • Loss of appetite
  • Ulcers
  • Tiredness
  • Skin redness

If the disease is diagnosed early enough, it can be treated by using anti-bacterial medications. However, these types of medications are not very effective for fish who are suffering from severe tuberculosis. There is no 100% effective treatment for this disease.

The best chance of success for treating tuberculosis in marine life is by keeping infected fish away from other occupants in the tank.

Tuberculosis thrives in saltwater tanks that are poorly cleaned and maintained. Bad water quality and having too many occupants in the tank can also increase the chances of the bacterial infection growing and spreading among the fish.

It is also important to keep in mind that this is one of two saltwater fish diseases that can be passed to humans from fish.

This is one of the reasons why it is important you use caution when you are treating and handling saltwater fish who are infected with tuberculosis.

Fin Rot

This is a bacterial infection that affects saltwater fish who are either being bullied by other fish in the tank or are living in poor tank conditions.

When there is aggression among tank mates, fin-nipping is a very common behavior. Fin-nipping can lead to secondary infection, fin rot.

Symptoms commonly associated with fin rot are:

  • Reddening of fins
  • Fraying of fins
  • Loss of fins

The goal of the treatment is to stop the progression of the fin rot before the fish loses the entire fin. Any fish who are infected should be quarantined and given anti-bacterial medication for treatment. I recommend using Mardel Maracyn 2 as treatment. 


Water changes and regular maintenance routines are crucial in the prevention of many fish diseases. Always be proactive and keep a healthy environment for your fish.

Poor water quality and stress are common contributors to saltwater fish diseases.

Have a quarantine tank available to be used as a hospital tank or to house new fish before placing them in the main display tank.

Even though you cannot always protect your marine life from all saltwater fish diseases, you can reduce their chance of getting sick by following a basic tank maintenance schedule.

Early recognition of the symptoms will allow you to quickly begin treatment which increases the chance of saving the life of your fish.

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