It Marine Velvet. When a fish has marine velvet you will notice a fine dusting or powdering on the fish.
A fish infected with marine velvet may refuse to eat, hold its fins close to its body, and may scrape itself on rocks in an attempt to disloge the parasite. The fish may gasp for air and hang out at the tanks surface where the dissolved oxygen content is higher. Usually the gills are affected first and so the first symptom you may notice is increased respiration.
One treatment you can try, especially if only one or two fish are affected, is to give your saltwater fish a freshwater bath. The idea behind this is that the parasites will rapidly absorb lots of water through osmosis. Through the process of osmosis, water moves from areas of higher concentration to areas of lower concentration. Because the marine velvet parasite contains lots of salt, but not much water, this causes the parasite to fill so rapidly with water that it will burst when it is placed in freshwater.
To set up a freshwater bath you add 85% freshwater and 15% water from the aquarium. The temperature and pH must be matched to that in the aquarium. Also remember to use a dechlorinator.
