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Velvet in Saltwater Fish

Velvet disease in saltwater fish is caused by the protozoan Amyloodinium ocellatum. This organism has many similaries to, and is related to, the causitive agent of freshwater velvet (Piscinoodinium).

Like the freshwater velvet protozoan, the saltwater velvet organism contains chlorophyll and so it is often classified as an algae. The parasite produces some of its nutrients through the chlorophyll. This is why darkening your aquarium during a velvet outbreak is helpful in treating the disease. Chlorophyll is only active in the presence of light. By dimming the light you are cutting off the parasites food source (one of them anyway).

Marine Velvet Symptoms

If your saltwater fish have marine velvet you will notice a fine dusting or powdering on the fish. This dusting is actually comprised of many tiny whitish-yellow spots.

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.

Marine Velvet Life Cycle

The marine velvet parasite has two life stages. There is a free-swimming form, during which it is able to infect fish. There is also a cyst form in which the parasite undergoes a reproductive phase.

During the free-swimming stage the parasite 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. This cyst falls off of the fish and then undergoes reproduction by cell division for about three days. At the end of this period the cyst releases hundreds of new Amyloodinium ocellatum of the free-swimming form. These go in search of a new host. If one isn't found the parasite dies within 2-3 days. Unfortunately, in the aquarium new hosts are readily available. When the free-swimming form finds a new host the cycle begins again.

This is why if you find a saltwater fish infected with velvet you should place the fish in its own tank to hopefully prevent the spread of the parasite to your other fish (however, it may be too late by the time you realize your fish has velvet).

Marine Velvet Treatments

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.

It is possible that your fish may go into shock. Don't leave your fish unattended during a freshwater bath and don't leave your fish in the bath for more than 10 minutes. If your fish is in distress you should remove it sooner than 10 minutes. The fish may lay on its side while in the bath, but if it appears to really be in distress get it out immediately and place it back in the aquarium. Most fish don't last the entire 10 minutes and so be prepared for this.

Keep in mind that even if you give your fish a freshwater bath that there are likely to be more parasites present in the aquarium and so you're going to need to do something to prevent reinfection. The main purpose of giving your fish a freshwater bath is to provide some immediate relief to the affected fish.

Copper sulfate is an effective treatment for marine velvet, however, some fish such as clownfish, bannerfish, and other saltwater fish don't tolerate copper sulfate very well. Symptoms of copper poisoning are popeye, rapid respiration, and balance problems.

In addition, copper sulfate will kill off the organisms living on live rock, invertebrates and live coral, and algae in the tank. This is why it is best if you can isolate the affected fish and treat it in its own tank.

If you need to treat the entire tank and you have invertebrates you can use an ultraviolet sterilization unit. The ultraviolet light will kill some of the free-swimming velvet parasites, while leaving the invertebrates intact.

It is only the free-swimming form of the velvet parasite that is affected by either the UV light or copper sulfate treatment. The cyst form isn't affected by either treatment.