Fish Diseases – A Helpful List of What To Watch Out For!

Jennifer Doll

Jennifer Doll


Fish Diseases

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As hard as you may try, there is no absolute way to prevent disease and illness from affecting aquarium fish. 

Parasites, bacterial infections, and fungal diseases quickly prey on newly added, weakened, or stressed fish. There is no way to prevent some fish from becoming sick, but there is usually a way to get your fish on the right track to a full recovery. 

Keep reading to learn everything you need to know about fish diseases, how to identify them, and the best ways to treat your sick fish! 

How Do You Know if Your Fish Is Sick?

A sick cat or dog is easy to notice and usually straightforward to diagnose. A sick fish, though? It can be hard to determine the problem.

A good aquarium keeper will check on their tank at least once a day. Not only is this for viewing pleasure, but it also gives time to look for any changes in the tank or its inhabitants.

A sick fish will quickly display symptoms. Unfortunately, by the time symptoms become apparent, the fish – and usually, the rest of the tank – is already severely affected.

Some of the most telltale symptoms to look out for are:

  • Lethargy
  • Loss of appetite
  • Abrasions or tears in fins
  • Flashing/itching
  • White/black spots

Sometimes, fish won’t show symptoms until they’re close to dying. This is especially true if there are many fish in the tank.

What Are the Most Common Fish Diseases?

Aquariums are dirty. Good and bad bacteria are always present in the water column, and disease and infection can spread quickly.

Aquarists have struggled with parasites and bacterial and fungal infections for decades. So much so that you should assume that new fish entering the aquarium already have some type of ailment, making quarantine a necessary step.

A few diseases are the most common to come across, though. 

Because they’re so common, they’re usually the most treatable types of aquarium disease. Still, most common diseases need some treatment and will not go away on their own.

Here are some of the most common fish diseases and how to identify and treat them.


Ich is a parasitic infection that quickly spreads through aquarium systems.

It is often diagnosed as white spots covering the fish’s body, though these parasites can also lead to lethargy, difficulty breathing, and flashing. 

Unfortunately, there are two different kinds of ich, freshwater ich and saltwater/marine ich.

Freshwater Ich

Freshwater ich is caused by a parasite called Ichthyophthirius multifiliis. These parasites enter the system of the fish, quickly affecting the skin and the gills. 

Some fish can fight off contracting freshwater ich if their immune system is strong and water quality is good. However, poor water conditions and external stresses can cause a whole tank of fish to contract ich within a few days. 

If ich is already on one fish in your display aquarium, then all fish have already been affected.

Freshwater Ich Treatment

Luckily, freshwater ich treatment is relatively simple – as long as you don’t have a resistant strain.

The best way to approach ich is by raising the water temperature, performing water changes, and continually improving water conditions:

  1. Steadily increase the water temperature to 86° F (30° C). Keep in mind that some more sensitive species may be unable to handle this increase.
  2. Add additional oxygenation to the aquarium. Warmer water is less able to hold oxygen, so an air stone or powerhead is necessary to keep gases moving throughout the system.
  3. Perform a 25% water change every two days. Make sure to vacuum the substrate as the parasites will fall off the fish.
  4. Continue this process until 1-2 weeks after symptoms have subsided. If you fail to eradicate all the ich in your tank, it may come back.

Unfortunately, some strains of ich have become resistant to high temperatures and medications. 

Many medications can be used to treat ich, like Ich-X and ParaGuard; most of these medications are incompatible with beneficial bacteria and invertebrates, so they will need to be dosed outside of the main display.

Otherwise, follow the instructions and the fish should start to improve.

Saltwater/Marine Ich

Unfortunately, marine ich is much more difficult to treat than freshwater ich. Marine ich is Cryptocaryon irritans

Oftentimes, marine ich presents as many more, smaller white spots that coat the whole body of the fish. There are few success stories of treating marine ich in the display aquarium and a quarantine system is usually mandatory.

Some of the most effective treatments for marine ich are copper, chloroquine phosphate, tank transfer method, or hyposalinity. 

Each of these methods needs to be done outside of the main aquarium. To fully eradicate marine ich from the display, the tank must lay fallow for at least 76 days. 


Flukes are another parasite that is common in the aquarium hobby, though more so in saltwater environments. These parasites are Monogeneans, making them a type of flatworm.

There are several different types of fluke, though the most common symptoms of all flukes include yawning, flashing, cloudy eyes, and discoloration. Verified identification of flukes can be tricky, though. 

The problem is that flukes are transparent, making them invisible to the naked eye when attached to a fish. For saltwater fish, the best way to confirm a fluke infection is by dipping the fish in freshwater.

After a few minutes, the flukes will become cloudy and drop off of the fish. A microscope may be needed to identify the exact species of fluke you’re dealing with.

Treatments include dosing praziquantel and prazipro as well as hyposalinity, formalin dips, or freshwater dips if flukes are only present on the exterior of the fish. Wrasses can be especially sensitive to some of these medications. 

Freshwater medication treatments are the same but, unfortunately, hyposalinity and freshwater dips are not viable options.


Yet another type of freshwater and saltwater parasite, velvet is often misdiagnosed as ich. Unfortunately, velvet kills much faster than ich so time is of the essence.

Freshwater velvet is Oodinium pilularis or Oodinium limneticum, while saltwater ich is Oodinium ocellatum.

Freshwater and saltwater symptoms of velvet include scratching, flashing, lethargy, sensitivity to light, and heavy breathing.

Sometimes the fish will develop a thin layer or coating of white, yellow, or rust-colored film on its body; this can lead some hobbyists to confuse velvet with ich.

If you are able to count the dots, then you’re dealing with ich. If the dots are very close together and in large quantities then it’s most likely velvet.

Treatment for velvet needs to be aggressive as this parasite can move fast.

Chloroquine phosphate and copper may be dosed outside of the display fish tank. Saltwater fish may be given regular freshwater dips. It will also help to dim the lighting to keep your fish comfortable and to help stop the spread of the parasite.

Bacterial Infection

Bacterial Infection

Bacteria is all throughout the aquarium, both good and bad kinds. Though some bacteria might be considered bad, most of them are needed to sustain a balanced ecosystem.

Most of the time, healthy fish are able to fight off these harmful bacteria.

That being said, weakened fish can quickly succumb to these species, causing a bacterial infection to form. These are caused by either gram-positive or gram-negative bacteria.

Bacterial infections are often a result of another problem in the aquarium. This could include poor water quality, stress, or another injury or disease; it is very common for fish to develop an infection if they have been bitten or scratched by another fish or invertebrate.

Common symptoms of a bacterial infection include redness and inflammation around open wounds, frayed and breaking fins, cloudy eyes, and bloating.

Like humans, bacterial infections can be treated with antibiotics. Most hobbyists only like to use these medications as a last resort as they can be very powerful and difficult to dose.

In worst-case scenarios, medicines, like erythromycin and kanamycin, can be dosed under close supervision. Unfortunately, it can be very difficult to prescribe the correct antibiotic for any given infection and multiple tries are often needed to get the right one.

One of the best ways to treat a bacterial infection is by improving the immune system of your fish. Fish largely succumb to bacterial infections due to environmental or prior health problems. Fixing these issues will get your fish on the right track to recovery.

This includes improving water quality, increasing the quality of diet, and removing any problematic tank mates. Good fish husbandry practices may also be employed, like using a quarantine fish tank and running a UV sterilizer.

Fungal Infection

A fungal infection is much less common to come across and can be difficult to identify at first. 

Like bacterial infections, fungal infections are usually a secondary result of poor water quality, bad diet, or injury. Many fungi can arise if something has died or decomposed in the tank.

Symptoms of a fungal infection are very similar to those caused by bacterial problems:

  • Redness and discoloration
  • Cotton-like bodily growths
  • Flashing, itching, lethargy, lack of appetite

Some of the most common strains of fungi arise from Saprolegnia spp. and 
Ichthyophonus hoferi.

Because water quality is the leading cause of fungal and bacterial diseases, it’s recommended to perform large, consistent water changes until conditions improve.

In the meantime, some medications, like API Fungus Cure and kanamycin, can help improve symptoms.

Fin Rot

Arguably the most common example of fungal and bacterial disease is fin rot.

Fin rot is usually caused by bacteria from the Vibrio, Aeromonas, or Pseudomonas genus, but can also be the result of a fungus too. 

Again, fin rot is usually the result of poor tank and water conditions. Improving the quality of life for the infected fish through a better diet and increased water changes is usually the best course of treatment. 

In extreme cases, aquarium salt and other medications may be used to treat fin rot. However, this course of treatment should only be used in emergency situations as medications can stress the fish, leading them to a premature death.

For an in-depth guide about fin rot, make sure to check out our article here

Hole in the Head (HITH)

Hole in the head disease is probably the most frightening fish illness to deal with. It is very common to find in discus fish (Symphysodon spp.).

The main symptom is that the infected fish develops one or more scrapes or pits across its face; sometimes these may occur around the rest of the body, though this is a rarer symptom. Another common symptom associated with hole in the head is loss of appetite.

Hole in the head is caused by a naturally occurring protozoan from the Hexamita genus. A weakened fish becomes infected by these parasites, which then compromise the sensory pores in the face, causing damage to the surrounding tissue.

One of the most effective treatments for hole in the head is metronidazole. This medication is often dosed through food, though administration can be difficult if the fish refuses to eat.

At the same time, environmental conditions and diet should also be improved.

Pop Eye

Pop eye is one of the easiest fish illnesses to diagnose, but also one of the most painful-looking ones for your aquarium fish.

Pop eye sounds like what it is: a swollen, bulging eye or eyes. The eye is often cloudy and distorted, which can lead to eventual blindness if not corrected.

This is usually a secondary bacterial infection, though eyes can be swollen due to certain vitamin deficiencies, injury, tumors, and poor water conditions; if only one eye is affected, then the cause is likely due to an injury.

Epsom salt baths can be very effective for mild cases. If you’re dealing with an infection, then Maracyn 2 and erythromycin may be dosed alongside the baths.

Swim Bladder Disease

Swim Bladder Disease

Swim bladder disease is one of the hardest to watch your fish go through. The symptoms of this disease are very physically debilitating and treatment is difficult.

Like most of the other illnesses on this list, swim bladder disease is often a secondary infection; in order to treat your infected fish, you must first treat the initial problem.

Swim bladder disease is relatively easy to diagnose. Affected fish will have difficulty maintaining buoyancy in the water and might swim on their sides or upside down entirely.

Other symptoms include a bloated stomach and favoring the top and lower portions of the fish tank.

Swim bladder disease happens when the swim bladder, one of the important internal organs of fish, stops working properly. The exact causes are not known.

It is believed that poor water quality, constipation, digestive issues, or other fungal and bacterial infections can lead to problems with the swim bladder. These conditions need to be treated first.

In extreme cases, hobbyists construct floats to keep their fish swimming upright. This can help prevent cuts and scrapes from aquarium equipment and the substrate, though the wrong materials can injure the stress coat layer of the fish.

Unfortunately, the symptoms that come with swim bladder disease can sometimes be permanent and the fish may be forever disabled. Other times, the fish can make a full recovery.

Catching symptoms early on and correctly diagnosing the initial problem will give the best chance of a full recovery.


Dropsy can sometimes be misdiagnosed as swim bladder disease due to infected fish developing a distended abdomen. This disease is very commonly seen in goldfish.

Again, dropsy is usually the symptom of a bigger problem at hand, like poor water quality, prolonged levels of stress, and other infections. 

Dropsy is a type of bacterial infection that attacks fish with weakened immune systems leading to fluid buildup in the internal organs and cavities. As a result, the fish develops a bloated appearance with bulging eyes, clamped fins, and discoloration.

One of the most telltale signs of dropsy is the development of deformed scales that stick out away from the body of the fish, causing a prickly appearance.

Unfortunately, dropsy is a deadly disease that is difficult to treat. Immediate diagnosis and treatment are crucial for the recovery of diseased fish.

Medications that are designed to treat gram-negative bacteria, like Maracyn 2, can be very effective at diminishing symptoms. Aquarium salt can also be dosed at the same time in a quarantine system.

Preventing Aquarium Disease and Illness

Unfortunately, many hobbyists only learn how to deal with aquarium diseases and illnesses due to having an outbreak in their tank. When this happens diagnosis and treatment often come too late, leaving a beginner hobbyist with an empty tank.

While there aren’t any ways to 100% prevent the disease from entering the aquarium, there are definitely ways to make the chances of it happening incredibly slim. 

Here are some of the best ways to prevent diseases from entering your home aquarium:

Purchase from a reputable buyer.

This might seem simple, but many hobbyists opt for cheaper fish. A reputable buyer will likely have slightly higher prices, but this should guarantee that the fish is better prepared for success once in the home aquarium. 

Healthy fish will be active, bright, and eating.

For especially difficult or sensitive fish, it’s recommended to ask the store associate to see the fish eat. If the fish doesn’t accept food in the store, it’s highly likely that the fish won’t eat in your aquarium either.

Quarantine fish.

Nobody wants to quarantine their fish. Everyone wants to introduce their new aquarium fish to their tanks as soon as possible.

Not only are you introducing new fish by doing this, but you’re also introducing a plethora of unknowns into your aquarium as well.

A 2-4 week quarantine period is one of the best ways to catch diseases and illnesses early. A separate tank also gives a large amount of flexibility for treating those ailments as you don’t need to worry about affecting other organisms in the aquarium as you would for the display tank.

Quarantining fish is a sure way to make your fish as healthy and as strong as they can be before introducing them to your final setup.

Have medications on hand.

It never fails that an emergency happens when stores are closed. Disease and illness move fast through the aquarium and time is of the essence.

It’s better to be prepared than to have to wait to get to the store the next morning.

Having a handful of helpful medications can sometimes be the matter between life and death. It’s also always important to make sure that medications have not expired.

Water parameters and tank maintenance.

Aquarium water chemistry is always changing and no two days are the same. Even though an aquarium might be fully stable one day, it can collapse the next.

Taking routine water tests can help prevent poor water quality. Regular tank maintenance will further help keep conditions favorable so your fish have strong immune systems.

Daily checkups.

Lastly, you want to check on your fish every day. Disease and illness can appear overnight and you want to take note of major changes from one day to the next.

Not only do daily checkups help keep your fish healthy, but you also get to enjoy the hard work of owning a fish tank!

How To Treat Sick Fish

If your fish do manage to get sick, then you will need to know how to treat them. Every illness is different though, and it can be hard getting the correct treatment on the first try.

The best thing you can do for a sick fish is to place it in a quarantine tank as soon as possible. This will prevent other fish from stressing out the sick fish, which could lead to a quicker death. Quarantine also provides a safe space for observation and treatment. 

Unfortunately, quarantine does not stop other fish from becoming sick. By the time a sick fish shows symptoms, often times the entire tank is already infected.

 Luckily, a quarantine tank is relatively easy to set up.

How To Set up a Quarantine Tank

How To Set Up A Quarantine Tank

A quarantine tank should be large enough to fit a large fish or a school of smaller fish. This tank can be a traditional glass aquarium, but many hobbyists also successfully use clean plastic alternatives.

No substrate is needed as some parasites rely on a substrate to complete their life cycles. PVC piping may be used to provide the fish with shelter.

An adjustable aquarium heater and accurate thermometer are necessary for changing water temperature for some disease treatments. An air stone may also be added to help increase oxygen levels as some medications can cause decreases in oxygen.

Most importantly, this tank needs to be fully cycled with mature biological filtration. But how can you guarantee an established system if it’s a temporary setup?

This takes some planning ahead of time.

One of the best ways to be prepared for a quarantine situation is by having extra filter media. Simply add some extra filter media to your current filtration system that can later be removed in order to use in the quarantine tank. 

In most cases, this will almost instantly cycle the quarantine aquarium, making it safe to treat sick fish. 

It should be noted that many aquarium medications kill beneficial bacteria which can make maintaining water quality very difficult. Even then, the quarantine aquarium needs to be seeded with mature filter media upon start up.

Can Humans Get Diseases From Fish? 

Fish Diseases

Parasites and infections can quickly cause fish to die. But do aquarium keepers need to be worried at all while working in and around the tank?

In general, there is no reason to be concerned about working in your fish tank while your fish are sick. 

However, some illnesses, particularly bacterial ones, are zoonotic, meaning they can move between species. This is when it can become dangerous.

Do not work in a diseased aquarium or a medicated aquarium with open wounds. Anything present in the water has the possibility of entering the bloodstream and causing illness in humans, though this is very rare in the aquarium hobby.

Once finished working in the aquarium, thoroughly rinse your hands with soap and water. For extra protection, use aquarium-specific gloves.

It should also be noted that aquarium medications can be very strong and it is highly advised to use them only as directed.


There are many diseases and illnesses present in the aquarium hobby. Unfortunately, most of them are here to stay. It is up to the hobbyist to correctly diagnose their fish and proceed with the best course of treatment.

Fortunately, most fish can make a full recovery if the problem is caught early enough.

If you have any questions about freshwater or saltwater diseases, how to identify what’s wrong with your fish, or if you’ve struggled with one of the illnesses on this list, don’t hesitate to leave a comment below!

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