Symptoms of Velvet in Fish
Symptoms of velvet are similar to an ich infestation. You'll see small whitish spots on the fish, however, with velvet the spots will be much smaller and will not be completely white. In fact, the spots will be more yellowish in color. As with ich, the fish may act ill, with fins held close to the body, and you will often see the fish glancing off of rocks and other surfaces, presumably to dislodge the parasite, as they are irritating to the fish. If the gills are affected the fish will exhibit rapid respiration or gasp for air at the tank's surface.
Life Cycle of the Velvet Parasite
The velvet parasite has two life stages - a free-swimming form and a cyst form. The infective stage of this parasite is the free-swimming stage. During this stage the velvet parasite has 2 flagellae that enable it to propel itself through the water. The free-swimming stage of velvet is very small and is not visible to the naked eye (about 10 microns in diameter).
It propels itself through the water until it finds a suitable host, such as a fish. Once it finds a fish it will attach itself to the skin or gills of the fish and feed off of it.
Eventually it forms a cyst on the fish and remains there until it releases several hundred Piscinoodinium young that are of the free-swimming form. These go in search of another host and the cycle begins again. This is why velvet is so contagious.
Treating Fish Velvet
Fortunately, there are effective treatments for velvet. Copper sulfate seems to be the best treatment. You can use products with acriflavine in them, but these typically have unwanted side effects for the fish. The best product I've found for treating velvet is Coppersafe by Mardel, however, any copper sulfate solution made for aquarium fish should suffice. The only down side to using copper sulfate in your tank is that it kills invertebrates (if you have any).
The good thing about treating fish velvet with copper sulfate is that it also kills the ich parasite. Because of this, you don't need to distinguish between the two parasites. Copper sulfate gets rid of all external parasites in fish.
Keep in mind that it is only the free-swimming form of the velvet parasite that is affected by the treatment. In the encysted stage it is not vulnerable to treatment.
As mentioned earlier, if your fish have velvet, darkening your aquarium may help to irradicate the parasite. Adding a small amount of aquarium salt (1 tablespoon per 5 gallons of water) may also help.
Good luck!