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Mycosis (saprolegniosis)
Saprolegniosis is a fungal infection of the skin of guppies caused by aquatic molds of the genus Saprolegnia. These fungi occur naturally in fresh water, including in a well-maintained aquarium. They generally only settle on a fish that is...
What is it?
Saprolegniosis is a fungal infection of the skin of guppies caused by aquatic molds of the genus Saprolegnia. These fungi occur naturally in fresh water, including in a well-maintained aquarium. They generally only settle on a fish that is already weakened or injured. It is recognized by the appearance of tufts resembling cotton or wadding, white or grayish, on the skin, fins or sometimes eyes.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
The most visible sign is the presence of whitish or gray, cottony filaments attached to the skin, fins, mouth or eyes of the guppy.\n\nThe affected fish often becomes listless, remaining motionless in a corner of the tank or near the surface.\n\nIt may lose its appetite, rub itself against decor or plants, and present frayed fins or irritated skin under the areas affected by the fungus.\n\nIn advanced cases, open sores or areas of broken skin that do not heal may appear, especially where the fungus has grown on a pre-existing injury.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Saprolegniosis is caused by opportunistic aquatic molds, mainly of the Saprolegnia genus. These organisms are permanently present in fresh water and feed on decomposing organic matter.\n\nThey only become pathogenic when the fish is vulnerable: skin damaged by an injury, a fight, treatment against external parasites, or immunity weakened by stress or poor water quality. The fungus then colonizes the damaged area and gradually spreads.
Poor water quality (ammonia, high nitrites, dirty water) is the main risk factor for guppies.\n\nA water temperature that is too low slows down the fish's immune system and promotes fungal development.\n\nInjuries caused by fights between fish, abrasive decor, or rough handling open an entry point for the fungus.\n\nA fish already weakened by another disease, a parasite (such as white spot or worms), or significant stress (transport, overcrowding) is more likely to be affected.\n\nUnfertilized or dead guppy eggs in a breeding tank can also develop saprolegniosis, which can then spread to nearby fry or weakened adults.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis is primarily based on visual observation of the characteristic cottony filaments by the aquarist or exotic animal veterinarian.\n\nFurther examination may include sampling the filaments observed under a microscope to confirm the presence of fungal spores and distinguish saprolegniosis from other conditions that also cause a whitish appearance (such as certain bacterial infections).\n\nIt is important to also assess the water quality, as an underlying cause must be present. corrected in parallel with the treatment.
Treatment
Treatment combines the immediate improvement of water quality and the use of antifungal products specifically formulated for aquariums.\n\nBaths or treatments based on salt (sodium chloride) at low concentrations are sometimes used in addition, provided that the species tolerates them.\n\nIn certain cases, a local antiseptic treatment can be applied to the affected areas.\n\nThe precise choice of product and its duration of use must be discussed with a professional knowledgeable about aquariums, because certain antifungals are toxic to invertebrates or certain plants in the tank.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Maintaining clean water is the best prevention: regular water changes, effective filtration, and monitoring of parameters (ammonia, nitrites, nitrates).\n\nKeeping a stable and species-appropriate temperature avoids weakening the guppies' immune system.\n\nAvoiding overpopulation and limiting fights reduces injuries that serve as an entry point for the fungus.\n\nPromptly removing any dead fish, organic debris or unfertilized eggs limits the quantity of decomposing matter which feeds the mold.\n\nQuarantining any new fish before introducing it into the main tank allows you to identify a possible injury or infection before contaminating the group.
Possible complications
Without treatment, the infection can spread and destroy much of the skin or fins, paving the way for secondary bacterial infections.\n\nDamage to the gills or eyes can seriously compromise the fish's breathing or vision.\n\nIn severe cases, general weakening can lead to the death of the guppy, especially if the underlying cause (water quality, injury) is not corrected.
When to see a veterinarian
Appearance of white or grayish cottony filaments on the body, fins or eyes of the guppy.\nPersistent loss of appetite or unusual despondency of the fish.\nFrayed fins or wounds that do not seem to heal.\nSeveral fish in the same tank show similar symptoms in a short time.\nNo improvement after a few days despite an improvement in water quality.
Prognosis
The prognosis is generally good if the infection is detected early and the water quality is corrected quickly in parallel with antifungal treatment.\n\nOn the other hand, if the fungus has already invaded a large area of the body or reached the gills, the prognosis becomes more guarded, especially in a fish already weakened by another pathology.
Frequently asked questions
Is saprolegniosis contagious between guppies?
The fungus itself is found everywhere in the water, but it only settles on weakened or injured fish. A healthy fish next to an infected fish generally does not develop the disease, unless it is also compromised.
Can we treat an affected guppy without taking it out of the main tank?
Some general bath treatments are possible, but isolating the sick fish in a quarantine tank often allows for more targeted treatment and avoids exposing other fish to unnecessary products.
Is salt enough to cure saprolegniosis?
Salt can help as a supplement, but it is not a substitute for proper antifungal treatment or correcting the underlying cause such as poor water quality.
Should you completely change the water in the tank in case of fungus?
A regular partial water change is recommended to reduce the organic matter load, but an abrupt and total change can further stress already weakened fish.
References
AVMA — American Veterinary Medical Association
Content reviewed by the TunisieVet editorial team · Updated 11 August 2026
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