Home › Animalia › Animal health › Animal diseases › Velvet disease (Oodiniosis)
Velvet disease (Oodiniosis)
Velvet disease, also called oodiniosis, is a parasitic infection of the guppy's skin and gills. It is caused by a tiny single-celled organism, Piscinoodinium pillulare (close to golden algae). This parasite attaches itself to the skin of th...
What is it?
Velvet disease, also called oodiniosis, is a parasitic infection of the guppy's skin and gills. It is caused by a tiny single-celled organism, Piscinoodinium pillulare (close to golden algae). This parasite attaches itself to the skin of the fish and gives it a powdery, golden or rusty appearance, a bit like velvet. It is a common and feared disease in aquariums because it progresses quickly and can kill a fish in a few days if nothing is done.
Key point
A disease that can progress very quickly and cause the death of fish within a few days if not managed, rapid monitoring and action is essential.
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 characteristic sign is a thin, golden, yellowish or rusty veil on the body and fins of the guppy, visible especially under low lighting or a flashlight.
The fish rubs against plants, decor, or the bottom of the aquarium as if trying to relieve an itch.
It can remain folded in on itself, with its fins stuck to its body, and lose its appetite.
Respiration becomes faster as the gills are attacked, which can be seen by accelerated gill movements or the fish remaining near the surface.
In advanced forms, the skin may peel, the fins fray, and the fish becomes very weak, swims disorderly and then may die.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The disease is caused by Piscinoodinium pillulare, a flagellated parasite which has the particularity of containing pigments similar to those of algae, which also allows it to carry out some photosynthesis. It attaches to the skin and gills, feeds on the fish's cells, then detaches to multiply in the water in the form of cysts which release new infesting forms. This cycle can be very rapid, especially in warm, well-lit water.
A stressed or weakened guppy is much more susceptible to this disease.
The main contributing factors are poor water quality, an overcrowded aquarium, stress from transport or recent change of environment, water that is too hot, very strong and prolonged light which favors the multiplication of the parasite, and the introduction of new fish or plants not quarantined.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The diagnosis is mainly based on careful observation of the fish by the aquarist or veterinarian specializing in exotic animals, in particular using a flashlight which reveals the characteristic golden reflection on the skin.
A microscopic examination of a sample of mucus or a fin fragment can confirm the presence of the parasite and differentiate it from other skin diseases such as white spot disease.
As with any sick fish, it is recommended to have the water quality (physicochemical parameters) checked at the same time, because a water imbalance is often the cause or aggravates the disease.
Treatment
The treatment uses specific antiparasitics that can be used in aquariums, on the advice of a professional or a veterinarian specializing in exotic animals.
Reducing aquarium lighting during treatment is part of the management, as darkness or dim light bothers the parasite which needs light.
An improvement in water quality and a controlled slight rise in temperature may sometimes be advisable to speed up the parasite cycle and expose it more effectively to treatment, but this must be controlled as it can also further stress the fish.
You should never use treatment intended for other animals or improvise a dosage without specialist advice, as water and fish are very sensitive to concentration errors.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention relies above all on good management of the aquarium.
It is advisable to maintain stable water quality, not to overcrowd the tank, and to avoid sudden variations in temperature and lighting.
Any new fish or plant acquisition should go through a quarantine period before being introduced into the main aquarium.
Limiting the duration and intensity of lighting can also reduce the risk, since the parasite needs it to grow.
Regular aquarium maintenance, with regular partial water changes, helps limit the overall parasite load.
Possible complications
Without treatment, the infection can spread to the gills and cause severe respiratory distress.
Secondary bacterial or fungal infections can develop on skin damaged by the parasite.
In severe or neglected cases, the death of the fish can occur within a few days, particularly in already weakened guppies or fry.
When to see a veterinarian
Appearance of a golden, yellowish or powdery veil on the guppy's body.
The fish repeatedly rubs against the aquarium decor or floor.
Abnormally rapid breathing or fish staying near the surface.
Loss of appetite, depression or disordered swimming.
Several fish in the aquarium show the same signs at the same time.
Prognosis
The prognosis is good if the disease is detected early and treated quickly with a suitable product and improvement in breeding conditions.
On the other hand, the prognosis becomes reserved to poor if the disease is advanced, if the gills are severely affected, or if the fish is already very weakened, particularly in young guppies or fry.
Frequently asked questions
Is velvet disease contagious to other fish in the aquarium?
Yes, it is transmitted very easily from one fish to another through aquarium water. It is therefore important to monitor all the fish in the tank and not just the one that shows signs, and to treat the entire aquarium in general.
Can we see the parasite with the naked eye?
The parasite itself is too small to be seen with the naked eye, but its effect on the skin, this golden or powdery veil, becomes visible especially by shining a flashlight on the fish in a dark room.
Should the aquarium light be turned off completely during treatment?
A significant reduction in lighting is generally recommended because the parasite needs light to grow, but the precise modalities depend on the treatment used and should be discussed with a professional.
Can a guppy that has recovered from velvet get sick again?
Yes, a relapse is possible if favorable conditions persist, for example poor water quality or an overcrowded aquarium. It is important to correct the underlying cause in addition to treatment.
References
ESCCAP — European Scientific Counsel Companion Animal Parasites
Content reviewed by the TunisieVet editorial team · Updated 11 August 2026
AT A GLANCE
Need a veterinarian?
Find a practitioner near you on TunisieVet, with their specialities, hours and contact details.
Find a veterinarianCategories
Affected species
This information is provided for educational purposes only and is not a substitute for a veterinary consultation. If you have any concerns about your animal's health, consult a veterinarian.
To discover
Related diseases and linked Animalia content.