Marine Ich, Velvet, or Coral Fish Disease

Learn to Diagnose and Treat Oodinium

Fish in aquarium
Carlos Teixeira / Getty Images

What is Oodinium?

Marine ich, Velvet disease or Coral Fish disease is caused by an infestation of the parasite Amyloodinium ocellatum, which is a member of a large group of flagellated protists that are traditionally subdivided into two groups: the animal-like protozoa and the plant-like algae. Amyloodinium ocellatum belongs to the same group of single-cell alga organisms that cause red tides in marine waters. Although this organism does not cause red tides, it is parasitic on fish at one stage in its life cycle, which depending on the temperature of the water can be completed in six to 12 days.

Because this organism can reproduce so rapidly, when an Oodinium outbreak occurs in an aquarium and it is not immediately diagnosed and treated, being in a closed saltwater system, it can reach overwhelming and disastrous numbers quickly. This type of parasite is one of the most common causes of a wipeout, or an abrupt loss of all the fish in a saltwater aquarium.

The Life Cycle of Amyloodinium ocellatum

  • Free-swimming cells called dinospores are released from a mature cyst and go in search of a host fish. Typically these cells can survive seven to eight days without a host, but in lower tank temperatures at around 75-80 degrees, some strains may last up to 30+ days.
  • Once a host is found, typically heading for the soft tissue inside the gills first, the dinospores lose their swimming capabilities and become non-motile parasitic trophozoites. At this stage they turn parasitic, as each attaches to the host fish by sending out a filament for feeding.
  • After deriving nutrition for 3 days to a week the trophozoites become mature and may drop off into the substrate, may remain attached in the mucus membrane, or sometimes be deeply embedded in the tissue of a host fish, where at this point each forms a type of hard-shell covering.
  • Inside each encrusted cyst the cells, now called tomonts, reproduce internally by non-sexual division. Upon reaching maturity in about five days, each cyst ruptures and releases hundreds of new free-swimming dinospores to start the cycle all over again, but in much larger numbers.


The most common site of infection is the gills, so the primary clinical signs are respiratory. At the onset of this infestation fish often scrape up against objects in the aquarium, lethargy sets in, and rapid respiration develops, which is the result of excess mucus in the gills due to the invasion of the parasites. This is typically noticed as fish staying at the surface of the water, or remaining in a position where a steady flow of water is present in the aquarium.

As the disease progresses outwards from the gills, the cysts then become visible on the fins and body. Although these cysts may appear as tiny white dots the size of a grain of salt, like the first sign of Saltwater Ich or White Spot Disease, what sets Oodinium apart from other types of skin parasites is that at this point the fish appear to be coated with what looks like a whitish or tan to golden colored, velvet-like film, thus the name Velvet Disease.

Now in the advanced stage of the disease the production of gill and body mucus increases, the fish becomes listless, refuses to eat, and it's not unusual for a secondary bacterial infection to develop. For fish that reach this end stage of the disease, it's typically too late. They usually do not respond to treatment, and most often will die.

Most Effective Treatments for Oodinium

  • Remove all fish from the main aquarium, give them a freshwater dip or bath, and then place them into a Quarantint Tank (QT) with  vigorous aeration provided. Treat the fish in the QT with a copper-based medication per the manufacturer's product instructions. To address complications from secondary infections, also treat the fish with an appropriate antibiotic or anti-bacterial medication. Continue treating the fish in the QT until the parasite appears to be gone, and then keep treating for another week after that.
  • Unfortunately, Oodinium can withstand a broad salinity range (from 3 to 45 ppt) so Hyposalinity (freshwater dips) is not an effective treatment.

Preventing Reinfection

Reinfection will occur no matter how effectively the fish have been treated if the organisms are not eradicated from the main aquarium. Because they require a fish host to survive, this can be accomplished by keeping the tank devoid of any fish for at least four weeks. For fish-only aquariums hyposalinity can be applied during this period of time, and the tank temperature elevated to 85 to 90 degrees Fahrenheit to speed up the life cycle of the organisms, which will help to eliminate all cysts and dinospores in three weeks.

Several days prior to returning the treated fish to the main aquarium all filter equipment should be cleaned, filtering materials changed, and a partial water change performed.

Ich Medication Usage Tips

  • Porous materials such as sand, gravel, rocks and ornaments absorb copper. To better control the strength and effectiveness of copper-based medications, it is best to use a bare QT with only some pieces of PVC pipe added to provide shelter for the fish during the treatment period.
  • Although many over-the-counter remedies contain the general name as ich or ick treatments, carefully read the product information to be sure it is specifically designed to target "Oodinium".
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  1. Amyloodinium ocellatum, an Important Parasite of Cultured Marine Fish. Southern Regional Aquaculture Center, United States Department of Agriculture, National Institute of Food and Agriculture, 2020