The disease manifests as numerous fine spots on the fish's skin, fins and gills. Each tiny spot is a single parasite that appears green-yellow when viewed under bright light (hence its other common names: “Rust Disease” and “Gold-Dust Disease”). Its other popular name is “Oodinium” but this is rather misleading as Oodinium is in fact a genus of parasites of marine invertebrates!
The pear-shaped parasites anchor to the fish by root-like structures know as rhizoids which penetrate the skin and gills (see diagram). The parasite is a warm-loving organism that fares best at around 23-25oC.
Velvet disease has a wide host range. It is quite common in Killies, Anabantoids and Barbs but also infects other tropical fishes. It can be a real problem when rearing fish.
Similarities with White Spot (Ich)
The novice fish keeper might confuse Velvet disease with White Spot (“Ich”) as both diseases manifest as numerous small spots on the fish's skin and fins. They also cause similar symptoms (covered later).
The main distinguishing features, however, are the size and colour of the spots. The spots in Velvet Disease are only about 0.1- 0.2 mm diameter, whereas those of mature White Spot may exceed 1 mm diameter (hence 5-10 times larger).
Seen with the naked eye, a heavy infection of Velvet looks more like a dusting over the fish, whereas White Spot appears as discrete sugar-grain sized spots. In terms of colour, the spots of Velvet are yellow-green when seen under bright light, whereas those of White Spot are white-grey.
Interestingly, the Velvet and White Spot parasites have very similar life-cycles, which is remarkable considering these two organisms are not closely related (Velvet is a flagellate parasite whereas White Spot is a ciliate). Their life cycles involve a feeding and growing parasitic stage on the fish, followed by free-swimming and reproductive stages in the aquarium. The life-cycle of Velvet is shown in the diagram.
These depend largely on the severity of infection. During the early phases of a Velvet outbreak the affected fish may sustain only a mild infection (just a few parasites). The fish may show few if any symptoms, the early signs being fin twitching or body rubbing (flashing). As the parasites “cycle” and multiply in the tank, the fish will be exposed to heavier infections. Badly affected fish may develop areas of pale patchy skin (due to over-production of skin mucus), shimmying (= swimming on the spot), and loss of appetite. The fish may also suffer breathing problems due to parasite damage to the gills.
The fine dusting of spots should be obvious by now, but at this advanced stage the chances of survival are slim. As with many disease outbreaks, early detection and treatment is vital!
Velvet outbreaks in rearing tanks
Many fish breeders experience outbreaks of Velvet in their fry rearing tanks. This has led some to question whether the Velvet parasites could have come in via tap-water or via frozen or dried foods. In fact, none of these routes of entry are possible.
If Velvet occurs in the fry tanks then the Velvet parasites must already be in the aquarium (or fish house) from the start. Even wild-caught live foods (such as Daphnia, Bloodworms, etc.) are highly unlikely to carry Velvet as this is principally a warm-water parasite. So how could the Velvet parasites get into the fry tanks?
Bear in mind that the free-swimming stages of Velvet are microscopic (hence invisible to the naked eye) and can easily be passed from tank to tank via contaminated wet hands, nets, or via droplet spray. If Velvet is a recurrent problem when rearing fish, then think about moving the fry tanks well away from the main aquariums, to reduce the chances of spreading this disease around the fish house.
Wash your hands before working on fry tanks and keep a set of nets and other equipment solely for use on the rearing aquariums (ideally one net per tank).
Also bear in mind that Velvet can “tick over” at very low (hence sometimes undetectable) levels in your adult fish - especially in fish that have developed partial immunity to this parasite, arising from a previous exposure to Velvet. These semi-immune fish can harbour just a few Velvet parasites that you may not detect, and the parasite levels are so low that the fish may exhibit few or no outward symptoms.
Your fry, on the other hand, will not possess acquired immunity to Velvet and this may explain situations where you experience full-blown Velvet disease in the fry tanks but not in your adult stocks.
If you suspect Velvet in your adult stock then treat them with a suitable Velvet remedy before they are used for breeding. Parasitic stage remains anchored to the fish's skin or gills for about 3-6 days.
Velvet is reasonably easy to eliminate using one of the anti-Velvet cures on the market. For added effect, keep the aquarium in total darkness for 7 days.
This darkness trick works because Velvet parasites obtain much of their energy by photosynthesis (that's why they appear yellow-green, just like plants).
If denied light, the parasites will weaken and probably die. But don't rely on darkness alone to combat Velvet – it is much safer to combine darkness with a course of proprietary Velvet remedy from the aquatic store. Incidentally, this darkness trick won't work on Marine Velvet. This is because the Marine Velvet parasites (a different species: Amyloodinium ocellatum) do not photosynthesise.
Increasing the water temperature to about 28oC will speed up the parasite's life-cycle, enabling the chemical remedy to work faster. Personally, however, I am not in favour of this thermal manipulation as the fish may already be badly stressed by the disease such that an increase in temperature (which will boost the fish's metabolism and energy needs) could stress them further.
Leaving an aquarium completely fish-free for 7 days (at 25oC) will also rid the system of Velvet, as the parasites cannot exist off the fish for very long. (At lower temperatures longer periods are needed to disinfect the tank of Velvet.) Keeping the empty tank dark will also help, as discussed above.
Need further information?
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Last updated February 11, 2005