Learn about common fish diseases and how to prevent them by keeping your aquarium fish healthy.

Setting up your own aquarium can be a creative feat, but it also requires some skill and a lot of research–you’re building your own ecosystem. Typical freshwater aquariums are usually inexpensive to set up, and there are a ton of fish to choose from to fill your new aquarium, but you want to ensure that the fish species you choose are compatible to avoid any potential problems.

Having an aquarium can be an exciting experience and tempting for people who want pets but don’t want to put too much effort into taking care of them. It may seem simple to keep an aquarium–after all, unlike other pets, fish don’t actively demand too much of your attention–but maintaining your fish and keeping them happy can require a lot of attention to detail.

Unfortunately, aquarium fish are susceptible to bacterial disease. Continually monitoring your fish and the quality of your water will help you prevent these diseases. Still, there are some common diseases that tend to happen even in the most optimal conditions.

In this article, we explore some of the most common diseases among aquarium fish, explain how to treat them, and provide tips on keeping your fish healthy and preventing them from catching such diseases.

Although fish and aquariums can spread germs to people, there are few fish diseases that can directly cause illness to humans.

two betta fish in aquarium

Signs That Your Fish May Be Sick

The key to treating a sick fish is spotting when it is ill. You should be monitoring your aquarium daily to ensure that it’s in good condition and that each of your fish is behaving normally.

If your aquarium fish are showing any of the following physical signs or changes of behavior, they may be suffering from an illness:

  • Clamped fins: A condition in which a fish continually holds its fins against its body. Clamped fins are not indicative of one specific disease, but it is a clear sign that there is a problem with your fish or the tank, such as the presence of parasites or poor water quality.
  • Fish migrates to the top of the tank: this usually happens when a fish has trouble breathing; the oxygen concentration is the highest at the top of the tank, so the fish stays up there when the rest of the tank lacks oxygen. You may also notice that your fish’s gills are moving more rapidly than normal. If your fish exhibits this behavior, immediately check the oxygen levels of your tank.
  • Fish is less active: If you notice that your fish has become listless or lethargic, this may be a sign that their immune system is compromised. 
  • Change or loss of color: Depending on the color, this may be the sign of a disease in the fish.
  • Loss of appetite: If your fish is not eating, it could simply be a sign of overfeeding. If your fish continues not to eat, an illness may be internally impeding their appetite.

Preventative Measures

Actively preventing your fish from getting sick requires consistency and attention to detail, but it’s easier than treating an already sick fish. You also want to maintain your fish’s healthy immune systems to prevent them from catching a disease and give them the best fighting chance if they do end up sick.

Here are some tips to prevent diseases from entering your aquarium:

  • Quarantine: It’s always exciting to add a new fish to your aquarium, but to prevent also introducing new diseases, you should quarantine any new fish you purchase for at least two weeks before adding them to your aquarium.
  • Avoid overcrowding: It may be tempting to add more colorful fish to your aquarium, but the more fish in your tank, the more problems that may arise. Overcrowding causes stress in fish, which may weaken their immune systems. It may also encourage aggression from certain fish in the aquarium. Before adding any new fish to your aquarium, make sure that they’re compatible with the already there fish.
  • Inadequate filtration: Make sure that you are diligent about filter maintenance. Without proper filtration, your fish tank becomes a breeding ground for toxic build-up, which will make your fish sick and prevent them from breathing properly.
  • Check pH levels: Regularly check the pH levels and temperature of the water in your tank. Make sure there are never any detectable levels of chlorine, ammonia or nitrite present in the water.
  • Don’t overfeed: As a general rule, your fish should consume the food that you feed them within minutes of it being added. It may be tempting to add additional food to eat later, but any food they don’t consume could cause build-up in your aquarium and lead to unwanted bacteria. You should only serve your fish high-quality foods and check the expiration date of the food before you feed your fish.

General Treatments

It’s possible that you follow all preventative measures but still end up with a disease outbreak in your aquarium. Don’t stress! As much as we try to take care of our fish, bacteria still manage to sneak their way to the tank.

Every fish disease has its specific treatments, but there are a number of first steps that you can take once you recognize that one of your fish is sick:

  • Quarantine: This is both a treatment and a preventative measure. Once you recognize that one of your fish is sick, immediately remove the affected fish from the aquarium and treat them in quarantine in a hospital tank. In some cases, a disease is so infectious that you must assume that the entire tank and all tank mates are contaminated, in which case you must treat the tank as a closed system.
  • Decrease stress: Lower the amount of fish in your fish tank, and remove any overly aggressive fish that may be causing stress to the other fish in your aquarium. 
  • Salt: Salt aids in the osmotic balance of the fish by making the water salinity closer to the fish’s blood salinity. Add only one teaspoon per 1 gallon of water, and make sure that none of the fish in your aquarium are sensitive to salt. You can pour the aquarium salt directly into the aquarium or dissolve it in water first. 

Before treating your fish, it is recommended that you first consult with your veterinarian.

two goldfish in fish tank

Common Fish Diseases and Treatments

Fish can catch all types of diseases, but some are more common than others. 

Below is a list of some of the most common fish diseases found in aquariums. This list isn’t exhaustive but is a great starting point to familiarize yourself with the symptoms to watch out for in your fish and how to treat common illnesses.

Anchor Worms

Anchor worms, also known as lernaea, are macroscopic parasites, meaning you can see them with the naked eye. They are commonly found in many freshwater fish species, such as koi and goldfish, making it a common disease in aquariums. 

Anchor worms are young parasitic crustaceans that attach to fish and burrow under their scales. The parasites are female, and once they attach to the fish, they turn into a reproductively-active adult. The “worm” name comes from the shape of the extending female reproductive structure.

This disease is usually easier to diagnose than others due to the size of the anchor worms. However, you may not be able to see any juvenile larvae that can be present in your tank. Other signs of the presence of anchor worms include your fish scratching themselves against objects in the aquarium or points of inflammation in your fish.

Cause and Prevention

Anchor worms are usually introduced into aquariums by new, infected fish and can spread quickly. The best way to prevent anchor worms is by quarantining any new fish before adding them to your aquarium. 

Treatment

Please resist the urge to manually pull out the anchor worms in your fish. Although you can pull them off of certain fish with tweezers, one wrong move can end up harming your fish.

Your best bet to safely remove the anchor worms is having your veterinarian remove them by sedating your fish, making the process less stressful for the fish. You can also use organophosphates or diflubenzuron (dimilin) to treat an anchor worm problem in your tank, but be sure to add the right measurements and use extreme caution.

Body (Skin or Gill) Flukes

Skin or gill flukes are worm-like parasites that are approximately 1mm long that can be found in your fish’s gills or the surrounding skin. These creatures have little hooks on their bodies that attach to the fish and drill through their flesh, most commonly through their gills. These holes can lead to bacterial infections. These parasites are especially present in betta or koi fish.

There are two main species of flukes:

  • Dactylogyrus: A fluke species that affect the gills. This species reproduces by laying eggs.
  • Gyrodactylus: Skin flukes, although they are known to also attack the gills. This species reproduces by giving birth to live young.

Flukes in aquarium fish are very small therefore difficult to see with the human eye. To get a definitive diagnosis, a skin scrape or gill biopsy will have to be done. 

Signs that your fish may have flukes include:

  • Scratching against objects in the aquarium
  • Swollen gills
  • Cloudiness from excessive mucus production
  • Damaged gills

The accumulation of mucus is also a symptom of columnaris, so make sure you diagnose your fish correctly in order to give it the proper treatment.

Cause and Prevention

Flukes can be present in aquariums that are kept in good condition and be harmless to healthy fish. However, flukes thrive in undesirable environmental conditions, such as poor water conditions or overcrowding of your aquarium, which causes stress to the fish and makes them more susceptible to disease. Once the fluke attaches to one host, it can multiply and spread very quickly, at which point you’ll need to treat the entire tank.

To prevent flukes, keep your tank clean and avoid overcrowding your fish. Control the background waste levels of your tank to ensure the immune system of your fish remains functional.

Treatment

There are numerous treatments available to treat flukes, most of which only require you to add tablets to your aquarium. One of the most popular methods is using Praziquantel (Prazi), a gentle tablet treatment that is effective on both types of flukes. If your fish ends up getting a bacterial infection as a result of flukes, treat them for flukes first. 

Several treatments may be required depending on the severity of the disease. Gill flukes often require at least two treatments, as the parasite’s eggs are usually immune to medication and may hatch after the treatment has taken effect.

Columnaris

Columnaris is an external or internal bacterial infection that most commonly occurs in livebearing fish, such as guppies. Columnaris is also referred to by other names such as guppy disease, saddleback disease. The disease is often confused with fungal diseases such as cotton mouth disease. Columnaris can be acute or chronic.

Columnaris bacteria are column-shaped (hence the disease’s namesake) but microscopic, so you’ll have to look for other signs to determine if your fish has this disease. Different species of fish react to columnaris differently, but the disease is characterized by gill damage and change of color, usually to a white or cloudy color. 

If left untreated, columnaris will continue to eat at your fish’s scales, causing visible lesions which may eventually cause a hole in your fish.

Humans cannot catch columnaris.

Causes and Prevention

The bacteria that cause columnaris are already prevalent in all fresh water–they’re probably already in your aquarium. When this bacteria overtakes your aquarium and enters the fish through the gills, mouth or wounds, it becomes dangerous. Depending on the strain, columnaris can kill your fish within 24 hours, before you even notice that it was sick. 

To prevent columnaris, avoid overcrowding your aquarium and creating a stressful environment for the fish.

Treatment

Columnaris is a highly contagious disease and, if left untreated, can be fatal. If you think your fish may have columnaris, quarantine it immediately and check your aquarium’s water levels. Columnaris bacteria thrive in warmer temperatures, so slowly lower the water temperature to around 75 °F (24 °C), to decrease the spread of the bacteria.

You should then use chemicals such as nitrofurazone (Furacin) or an antibiotic such as kanamycin in the water to treat columnaris and give your fish the best chance of survival. You may have to repeat this treatment several times over a period of up to 10 days for it to be effective.

Adding a small amount of aquarium salt (1 to 3 teaspoons per gallon) to your tank is also effective in diminishing the stress caused to the fish by the bacteria.

tiger striped fish in aquarium

Dropsy

Dropsy also referred to as edema, is the swelling of soft tissue in the fish’s body cavity. This swelling is caused by the accumulation of fluids or renal failure. If your fish develops a huge swollen belly, that’s a good indicator that they may have dropsy. Dropsy causes your fish to appear lethargic, and they will eventually lose their appetite. This is a common fish disease for goldfish.

As the disease progresses, your fish may develop skin lesions, and their internal organs such as the kidney or liver may be damaged. In severe cases, the fish’s eyes might bulge, and its scales may protrude from the body. Unfortunately, some fish may show no outward systems at all, making this disease difficult to detect.

Behavioral symptoms to watch out for include:

  • Refusal to eat
  • Swimming near the surface of the tank
  • Lethargy

Fish who have dropsy also often have popeye.

Causes and Prevention

Dropsy is usually caused by a number of symptoms that end up compromising a fish’s immune system. Although many fish can be exposed to the bacteria that cause dropsy, they rarely affect fish with healthy immune systems. 

Factors that cause a fish’s immune system to weaken:

  • Overcrowding
  • Poor water quality
  • Extreme drop in aquarium temperature
  • Improper nutrition

To avoid dropsy, keep your tank’s water clean with filtration or water changes, avoid overcrowding, and don’t overfeed your fish.

Treatment

If you suspect that your fish has dropsy, quarantine them immediately. Treatment is only likely if the disease is determined early on. Otherwise, the chances of your fish’s survival are slim.

Once you quarantine the infected fish, add one teaspoon of aquarium salt per gallon of water to their tank to help the fish get rid of the excess water in their body. If your fish is still eating, keep feeding them high-quality foods. If not, treat your fish with an antibiotic, such as Maracyn-Two, over ten days or whatever your veterinarian recommends. 

Fish Ich (sometimes called Ick)

Fish Ich is one of the most common fish disease among freshwater fish, and it is the disease that you are most likely to encounter in your fish tank. It is often referred to as “white spot disease” due to the visible white spots it causes on the body and fins of infected fish. Sometimes the white spots are too small to see and therefore need to be examined with a microscope. 

Infected fish may become irritated and will scratch themselves on objects or at the bottom of the tank. The disease may also cause respiratory failure and loss of appetite, which can lead to death.

The life cycle of ich is split into three stages:

  • Parasitic trophont
  • Reproductive tomont
  • Infective tomites released by the tomont

The last phase sees the ich ready to seek out fish to attach to, where it begins to feed on the fish and cause damage.

Causes and Prevention

Ich is caused when fish come into contact with protozoan, a parasite that is a single-celled organism and requires no intermediate host to reproduce. It’s thought that most species of fish have evolved and developed immune systems good enough to fight off protozoans, so fish with compromised immune systems are more prone to being infected with ich.

Because ich is so prevalent, it’s hard to prevent the disease. The best preventative measure you can take is quarantining any new fish before you add it to your aquarium and continue to maintain the water quality of your tank.

Treatment

If you suspect that ich is present in your aquarium, you’ll want to slowly raise the temperature of your tank to 78 F – 80 F to encourage the reproduction of the trophont, as ich can only be killed in the tomite stage.

To treat reproductive trophonts before they penetrate the fish, you can use a combination of formaldehyde or copper sulphate and malachite green.

You cannot kill ich while once they’ve penetrated the fish, so there’s no use in moving infected fish to a quarantine tank. If one fish has ich, assume that the entire tank is infected. To treat the tank, however, you can remove all of the fish and raise the temperature of the aquarium water to 80 F. Without fish to latch onto, the existing ich in the tank should die in a matter of days.

Fish Tuberculosis (Mycobacteriosis)

Fish tuberculosis is caused by a bacteria called mycobacterium, which is widespread in aquatic environments. 

This bacteria can infect mammals, birds, reptiles and fish and is zoonotic, so yes, humans can catch tuberculosis from their aquarium. To prevent this, try not to handle your aquarium while you have an open wound, as that is the most common way that this bacteria transmits from fish to humans.

Unfortunately, the symptoms of fish tuberculosis are non-specific, making this disease difficult to diagnose. Many fish are even asymptomatic, and the disease is not detected until they’re dead. Some common noticeable symptoms include:

  • Hollow belly
  • Loss of appetite
  • Listlessness
  • Swelling
  • Fin erosion

Cause and Prevention

Fish tuberculosis is usually caused by adding new infected fish or infected plants to your aquarium. The best way to prevent transmission is to quarantine all new fish before adding them to your aquarium. However, because the disease is so hard to detect, even if you do practice proper quarantine protocols, you may still end up introducing the bacteria that causes fish tuberculosis to your aquarium.

You may also transmit the disease to your fish. Mycobacterium is common in the environment and can be found on different surfaces, so if you handle your fish with your hands, you may transmit this bacteria to them and cause tuberculosis. To prevent this, always wash your hands before handling your fish. 

Treatment

Unfortunately, there is no fully effective treatment against fish tuberculosis. The best thing that you can do is to make sure that your fish remains comfortable in their tank, as some fish can continue to live with the disease for a long time. 

Due to the contagious nature of this disease, if one fish tests positive for tuberculosis in your aquarium, assume that all of the fish have been infected. At this point, you can continue to support your aquarium by treating it as a closed system. Do not add any new fish, and make sure to only use gloves when handling the aquarium. Otherwise, your fish will need to be euthanized. 

Fin Rot

Fin rot can refer to a symptom found in fish or an actual disease in and of itself. The condition is the result of a bacterial infection that causes the edges of the fish’s fin to discolor. As it progresses, pieces of the fin begin to die and fall off. Unfortunately, symptoms can be difficult to notice until the disease progresses to this stage.

Some symptoms of fin rot to look out for are:

  • Frayed fins
  • Inflammation at the base of the fin
  • White fin edge
  • A part of the fin is fully missing

Fin rot is contagious and can also cause secondary infections. It’s common for fish infected with fin rot to also be infected with columnaris. 

Cause and Prevention

Fin rot is usually caused by poor water quality that becomes a breeding ground for bacteria. Stress from movement, overcrowding and overly aggressive behavior from other fish can also cause fin rot.

Fin rot is common in aquariums, especially in betta fish and goldfish, but is also easy to prevent. As long as you maintain your aquarium by changing and monitoring the water and monitoring your fish’s behaviour, your aquarium should be safe from a fin rot outbreak.

Treatment

Fin rot is easy to prevent, and treatment is fairly simple. There are a number of antibiotics that are effective in treating the disease. Consult with your veterinarian to find the best one to treat your infected fish. If detected early enough, the infected fish’s fins should eventually grow back with no problems. 

If the fin rot becomes too advanced and reaches the base of the fin, it may be too late for the fin to regenerate, and the disease may begin to attack the fish’s body, resulting in eventual death.

If you suspect that one of your fish has fin rot, quarantine it and immediately change the water in the tank and check the filter. 

Swim Bladder Disorder (Flipover)

A fish’s swim bladder is the internal organ that controls its buoyancy. Swim bladder disorder is when the swim bladder does not function normally and impedes the fish’s ability to swim. It is an especially common disorder among goldfish.

The symptoms of this disease are fairly easy to spot. A change in buoyancy in your fish is a clear sign of swim bladder disorder–if your fish sinks to the bottom of the tank or floats to the top, then they may be suffering from swim bladder disorder. The disease may also cause your fish to float upside down or on its side. This lack of body control can affect the fish’s ability to eat. 

Cause and Prevention

Swim bladder disorder can be caused by compression of the swim bladder. This happens when the fish overeats or gulps air. Other problems with other nearby organs can affect the swim bladder and initiate this disorder. It can also be caused by constipation. Low water temperature can slow your fish’s digestive process, and bacteria can cause inflammation of the swim bladder. 

The best ways to prevent swim bladder disorder are:

  • Maintain a clean tank
  • Consistently check the water temperature
  • Feed fish only high-quality foods
  • Avoid overfeeding fish

Treatment

Treatment of swim bladder disease depends on the cause. If the cause is thought to be an enlarged stomach or intestines, then you should stop feeding your fish for three days and increase the water temperature to around 78 F. Once the three days are up, to counter constipation, feed the fish a green pea. You can do this for a few days, but fish generally cannot live off of peas alone, so be sure to eventually switch your fish back to its normal diet.

If this treatment does not work or swim bladder disease is caused by another infection, you may have to take your fish to a veterinarian so that they may manually remove the air or perform surgery on the swim bladder.

Popeye Disease

Popeye is a condition where one or both of your fish’s eyes are swollen and protrude from its socket. Some species of fish naturally have bulging eyes, so make sure your species isn’t one of them before diagnosing them with popeye. The infected eye may also appear cloudy or discolored in some cases–or even bloodstained. Popeye can affect any type of aquarium fish. 

Popeye is usually the result of fluid build-up accumulating behind or within the eye. The disease itself is not fatal, but it may make your fish more susceptible to other common diseases. If the infected eye ruptures, the fish may end up blind from that eye, or the entire eye may fall out. 

Cause and Prevention

If you suspect that only one fish in your aquarium is infected, and only one of their eyes is exhibiting symptoms, popeye could have been caused by injury from bumping into an object or aggressively interacting with another fish. This can be prevented by making sure all of the fish in your tank are compatible, minimizing any cause for aggression. 

If more than one fish in your aquarium are exhibiting symptoms of popeye, the problem may be poor water conditions in your tank. Test your water to see if there’s something unusual with its pH level.s 

Popeye can also be the result of other underlying infections caused by bacteria, fungi, or parasites. Popeye is common in fish that are suffering from dropsy, which usually results in a fatality. 

Treatment

If you think one of your fish has popeye, change the water in your tank right away and move the infected fish to a quarantine tank.

In the case of physical injury, the swollen eye will usually easily subside and heal itself, although you should continue monitoring the fish for infection.

Treat the infected fish with antibiotics recommended by your veterinarian. If there are multiple fish in your tank exhibiting popeye symptoms, you may need to treat the entire tank with antibiotics. Adding salt to the tank may reduce overall swelling. 

If popeye has led to another bacterial infection, you must address that as well using an antibiotic recommended by your veterinarian. 

Conclusion

Aquariums are fun to set up, but they’re also a lot of responsibility to maintain. 

There is an array of fish for you to choose from, and it’s always exciting to add a new fish to your aquarium. However, you want to make sure that all of your saltwater fish or freshwater fish stay healthy and get along with each other. 

As much as you can try to keep optimal conditions in your aquarium, things happen that may cause either one fish to get sick or an outbreak to occur. Luckily, if detected early enough, most fish diseases are treatable. Keep in mind that it’s easier to take preventative measures than potentially to have to euthanize one of your fish. 

If you suspect your pet fish is sick, your best course of action is to contact your veterinarian right away and always check with your veterinarian before treating your fish.

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