Velvet is one of the more common diseases in aquarium fish, and can quickly strike down every inhabitant in the tank before the owner realizes what he or she is dealing with. Also known as Rust or Gold Dust disease, it is caused by one of several species of a tiny parasite known as Oödinium.
Oödinium is a dinoflagellate, a creature classified by some as a protozoan and by others as algae because it contains Chlorophyll. Oödinium can occur in both fresh and saltwaterstrikes fresh and saltwater fish.
In freshwater fish Velvet is caused by either Oödinium pilularis or Oödinium limneticum. In marine fish the species is Oödinium ocellatum and causes Coral Fish Disease. All three species have symptoms and lifecycles similar to the well known parasite, Ich.
The single-celled parasite’s life cycle can be divided into three major phases. First, as a tomont, the parasite rests at the water’s floor and divides into as many as 256 tomites. Second, these juvenile, motile tomites swim about in search of a fish host, meanwhile using photosynthesis to grow, and to fuel their search. Finally, the adolescent tomite finds and enters the slime coat of a host fish, dissolving and consuming the host’s cells, and needing only three days to reach full maturity before detaching to become a tomont once more. They must find a host within 24 hours, or die.
Oödinium produces whitish/yellow pustules on the fish that are much finer than the spots seen in Ich. In fact they are so fine they are often not seen before the fish perishes.
Like Ich, Oödinium is present in most aquariums, but only becomes a problem when the fish are stressed by poor quality water, changes in the water temperature, or being transported.
Initially the fish rub against hard objects trying to dislodge the parasites. As the disease progresses the fish becomes lethargic, fins are held close to the body, appetite is reduced and the fish loses weight. A key symptom is difficult breathing, resulting in rapid gilling.
Perhaps the most telltale symptom is the appearance of a velvety film on the skin that resembles gold or rust colored dust. The film may be difficult to see, but can be more easily detected by directing a beam of a flashlight on the fish in a darkened room. The parasite is most often seen on the fins and gills.
Velvet attacks all fish and will even affect fry that are only a few days old. Anabantoids, Danios, Goldfish, Zebras, and Killifish are particularly susceptible to velvet disease.
Because Velvet is highly contagious and usually far advanced before being diagnosed, it is important to take steps to treat it as soon as possible.
Treatment is targeted at the free-swimming stage of the parasite.
Sodium chloride is believed to mitigate the reproduction of Velvet, however this treatment is not itself sufficient for the complete eradication of an outbreak. Additional, Mardel Treatments added directly to the fish’s environment include Copper Safe or Quick Cure. Additionally, because Velvet parasites derive a portion of their energy from photosynthesis, leaving a tank in total darkness for seven days provides a helpful supplement to chemical treatments. Finally, some enthusiasts recommend raising the water temperature of an infected fish’s environment to to 82°F, in order to quicken the life cycle (and subsequent death) of Velvet parasites; however this tactic is not practical for all fish, and may induce stress in some species.
CopperSafe is the treatment of choice. It should be used according to the recommended dosage for a full ten days to ensure that the parasite is completely eradicated.
As with any treatment, activated carbon should be removed from the filter, as it will remove the drugs from the water.
Velvet usually only arises when poor aquarium conditions prevail and is highly infectious. Quarantine of new fish for two weeks will greatly reduce the likelihood of contaminating a healthy established aquarium. Any fish that appear to be ill should immediately be removed and kept in a hospital tank to avoid the spread of the parasite.