What is velvet (or gold dust disease)?
Freshwater velvet is caused by external parasites from the Oodinium genus. Under a bright light, it looks like gold, dust-like particles covering the fish's head, body, and fins. It often starts in the gills, so the fish is very sick by the time you see external symptoms on the skin. Flashing, or sudden rubbing against surfaces, is another common symptom.
Unfortunately, this disease is quite difficult to treat, spreads rapidly, and does not respond to most broad-spectrum medications. Some hobbyists recommend using UV sterilizers and very strong, copper-based medications, which can be hard on an already weakened fish.
Because the parasite is photosynthetic and requires light to grow during its juvenile state, our founder Cory had some success treating his glass catfish by dosing the water with aquarium salt (regular sodium chloride rock salt, not marine salt) at a concentration of 1 cup of salt per 20 gallons of water. This translates into roughly 1 rounded tablespoon of salt per 1 gallon of water. Then he kept the aquarium in complete darkness for 7 days by turning off the light and covering it with a black trash bag, tarp, or other opaque wrapping material.
Note: some fish like anchor catfish are extremely sensitive to salt, so if you are not sure, consider ramping up the salt concentration to see how they react to it.