5 Most Common Fish Diseases

Detailed information about common aquarium fish ailments and how to treat them.

Aquarium fish can get sick for a variety of reasons: Stress, diet, parasites and/or bacteria being introduced to the tank, sudden fluctuations in water parameters, and more. This detailed guide provides an overview of 5 of the most common aquarium fish diseases and treatments. 

When in doubt, remember to consult a professional veterinarian:) 

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Five Most Common Aquarium Fish Diseases (And Treatments)

Aquarium fish diseases can occur for a variety of reasons: Some of the most common culprits are new bacteria that has been introduced (from not quarantining a new fish or live food, for example), poor water quality, and stress, which suppresses the immune systems of fish.

Examples of stress include sudden changes in water parameters, aggressive tank mates, poor diet and/or feeding schedule, overcrowding, or inversely, lack of tank mates (such as in the case of schooling fish).

Sometimes, aquarium fish may be living in suboptimal conditions with a limited volume of water and poor water quality, among other things. In the wild, fish can escape from harmful and infected surroundings. But in freshwater tanks or aquariums, they’re stuck.

We’ve created this comprehensive guide to common aquarium fish diseases to help identify what went wrong, and how to fix it. If your aquarium fish gets sick, we definitely recommend working with your trusted veterinarian and your local aquarium store to address and treat the illness.

Please note that this post is not medical advice and should not be taken as such; Work closely with your trust veterinarian so they can diagnose and treat your fish accurately.

1 Ich

Ich (ichthyopthirius multifiliis) is a common parasite in water tanks caused by protozoa (parasite). The parasites attach to the body of the fish and cause breathing disorders and mobility issues.


The apparent sign of Ich is tiny white marks or nodules on the body, gills, and fins. In addition to this, rapid breathing patterns, rubbing against a rough surface, or disorientated swimming in the aquariums also indicate Ich.


Ich is usually caused by a sudden drop in water temperature in winter or because, damaged heaters, or not quarantining a new addition to the tank (eg, when you introduce a new fish or plant in the tank which carries the protozoa).


You can use an anti-parasitic medication to treat the infected fish. Also, increase the temperature up to 86°F (if your fish can sustain in this temperature) to speed up the lifecycle of protozoa. Lastly, remove the carbon filter from your aquarium temporarily as it absorbs medication from the water.

NOTE: Be aware that it can be easy to mistake ich for fish tuberculosis- Which, by the way, can be transferred to humans. Wear gloves when handling fish that may be sick. Check out The Fish Doctor’s video below to learn more:


2 Dropsy


Dropsy is a common tank fish disease that can be relatively easily diagnosed. The body of the infected fish swells and in some cases the scales of the fish may overhang. The infected fish may show behavioral changes, which include disoriented swimming at the top of the water and refusing to eat food. (https://www.ratemyfishtank.com/amp/blog/behavioral-changes-and-problems-in-aquarium-fish )


Bacteria named aeromonas causes dropsy. Aeromonas is commonly present in tank inhabitants but it only affects fish in stressed environments. The kidney of the fish is affected by dropsy, which allows water to enter the body and results in swelling. (https://www.ratemyfishtank.com/blog/what-is-dropsy-and-how-do-i-treat-it )


Treatment of dropsy means also resolving the underlying issue. Here are some common methods for treating dropsy in fish:

An antibiotic formulated for the treatment of the dropsy bacteria is Mardel Maracyn 2 (it provides relief from bacterial infections). Shift the infected fish to an isolated “hospital” tank. Try to keep the temperature of the tank the same as the original tank. Saltwater (2.5 Magnesium Sulfate per 10 gallons) may help to remove excess water from the body of fish. Feed antibacterial fish food to the infected fish for about 7-10 days. If there is no sign of improvement, contact your vet.

If you can’t tell, I’m a big fan of Dr. Loh’s videos. I think they’re easy and helpful. Luckily, he has one on dropsy:


3 Fin Rot


Fin rot is indicated by black, brown, or white spots at the edge of the fish’s fins or body. The infected fish may refuse food and stop any sort of movement.


This is a common disease in aquarium fish caused by various species of bacteria and stressful environments. This together affects the immunity of the fish. Common reasons for a stressful environment are poor water quality, aggressive tank mates, overcrowding, and inadequate diet.


To treat fin rot, locate the cause of stress in the aquarium and resolve it to provide a healthy environment for the infected fish. Change 25% of the water to restore the water balance and improve water quality. Feed your fish with high-quality food in small quantities to avoid overfeeding.

When water changes and removing the stressors don’t resolve fin rot, there are a number of antibiotics available in pet stores that include Tri-Sulfa Tablets and Melafix among many other antibiotics available at pet store and from veterinarians. Consult your vet. After successful treatment, fins (ideally) regrow after a few weeks.

4 Velvet Rust


The signs of velvet dust appear in the behavior and appearance of the fish. In the early stage, the fish rubs its body against different surfaces. The yellow or gold spots on the body of the fish indicate velvet disease. In the advanced stage of the disease, fish shows rapid movement, weight loss, and loss of appetite.


This is a serious and contagious, common fish disease that is caused by a parasite (piscinoodinium). It attaches to the body of the fish and appears as golden dust or velvet. Yick!


Check the water quality and parameters and take necessary measures accordingly. Check levels of nitrite, nitrate, ammonia, and chlorine in the aquarium water. Change 30 to 50% of the aquarium water. It’s advised to switch off the aquarium lights as piscinoodinium is dependent on light. Also, work closely with your veterinarian to increase the water temperature so as to speed up the cycle of piscinoodinium.

Salt immersion dips are a common treatment, and there are some commercial medicines available that contain copper and quinine. Acriflavine and Acridine are much safer to use for fish as compared to other medicines available in the market. Work with your vet to determine the right option for your fish.

5 Anchor Worms


Anchor worms (lernaea) attach to the skin of the fish by using their hook-like claws. It feeds on the blood of the fish, and can be seen with the naked eye.


Lernaea is primarily caused by overcrowding, change in water temperature and condition, stress, and imbalanced water pH. The two most common causes of this infection are poor water quality, and infested carrier fish.


Here are common treatments for anchor worms: Placing the fish in a Potassium Permanganate bath to kill immature anchor worms on its body; Using Dimilin for 3-5 days to make sure the eggs and larvae of anchor worms are completely killed; Manual removal of worms. (With a pair of tweezers or a blade, the hanging head of the worm is removed.)

Other common treatments include saltwater dips, and performing water changes to give the fish a healthy environment in which to heal.


It’s extremely important to keep the environment of the aquarium healthy to prevent diseases and for the well-being of fish. The most common culprits of aquarium diseases are the different types of stress encountered by the fish, and the introduction of new bacteria or parasites caused by failing to quarantine new tank mates.

Stress encompasses poor water quality and/or diet, sudden fluctuations in water parameters, unsuitable tank mates (such as aggressive fish being placed with peaceful fish), overcrowding, or incorrect water parameters.

We recommend carefully observing your fish on a regular basis to keep an eye out for changes in feeding or behavior, and always quarantining new tank mates (including live food!) to prevent the introduction of disease.

If your fish does get sick, use a quarantine tank and work closely with your veterinarian to address the cause and treatment.