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Common Fish Diseases

Sometimes your fish will encounter a variety of diseases that is often hard to identify. Here we will teach you to become the doctor and help you identify the most common diseases found in fish along with recommended treatment and methods to prevent future outbreaks.


One of the most common types of fish disease are fungal infections. Fungal spores are always present in all aquariums whether it be freshwater, saltwater, cold temperatures or warm temperatures. Fungi, unlike plants are incapable of creating their own food by means of photosynthesis. Therefore they need a host whether it be living or deceased for growth and reproduction. Fish that may be stressed/weakened by disease, have open wounds, or in a poor water quality environment are very susceptible to fungal diseases.

Cotton Wool Disease is the most common fungal infection that infects externally to the fish's skin, fins, and mouth. It can be characterize by a fluffy white cotton wool like growths. If left untreated, it could spread and ultimately lead to death. Fungal diseases are highly infectious and may quickly spread to other fish that are in contact. We recommend quarantining for treatment.
Treatment may consist of a freshwater salt bath or the topical anti-fungal agent Gentian Violet. Many people have found success using Kordon's Malachite Green

Egg Fungus can be a nuisance for aquarist who are actively breeding their fish. This type of fungus is present in most aquariums and will infect damaged, diseased, or infertile eggs. This fungus can then spread quickly to healthy eggs. Naturally most fish will protect their eggs and remove all infected and diseased eggs, but sometime you will encounter a fish that will completely abandon their nest. At that point, it is recommended you manually remove all infected eggs yourself using a tweezer, needle or pipette. Many breeders will often use medication such as Kordon's Methylene Blue to help control fungal infections on eggs.

Gill Rot Disease is not a very common fungal disease but if you fish contracts it, it could be very deadly. Symptoms in an infected fish include difficulty in breathing, heavy gasping for air, discolored and eroding gills, fish hovering toward the top of aquarium gasping for air, and fish being very lethargic.
Gill Rot is caused as a result of low oxygen content in the water as well as high levels of ammonia and nitrate. These poor water conditions allow the fungus Branchiomyces to grow infect fish that are weak and high in stress.
While treatment is more difficult for infected species, we recommend an immediate water change to increase oxygen content as well as providing removing excess debris to provide cleaner waters. Increasing your oxygen level will help your fish breath easier. Treatment may also be attempted with medications such as Formalin.


In your aquarium you will always have the good and the bad bacteria. The good bacteria can be found in your biological filtration system or your gravel bed. This form of bacteria helps break down harmful ammonia and nitrate making your aquarium safer for your fish from harmful chemical. Then there is the bad bacteria floating in your aquarium ready to cling onto your fish that is either weakened, injured, stressed, or suffering from another disease. Once they find a fish that is vulnerable they will attack and spread extremely quick.

Fin Rot Disease
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