White Spot Disease in Saltwater Fish

betta fish in a saltwater aquarium

DEA / C. DANI / Getty Images

White spot disease, caused by the protozoa Cryptocaryon irritans, is a common parasite in saltwater aquariums. Since it only takes one parasite to reproduce into one thousand offspring, it is very easy for a system to get quickly overwhelmed. Replicating within gill tissue, this parasite is very harmful to your fish. Once you have a confirmed diagnosis, successful treatment must be carefully applied in order to eliminate it entirely.

As the name suggests, the main clinical sign of white spot disease is pinprick-size white spots along the body of your fish. They can be very hard to see in lighter-colored fish and are easily confused with lymphocystis or fin ray fractures.

Signs of White Spot Disease in Saltwater Fish

Other clinical signs include sudden death, lethargic fish, increased respiration and congregating around areas of higher water flow, such as powerheads, filter outflows and aeration. This parasite feeds in gill tissue, damaging the gills and reducing the fish's ability to take in oxygen, leading to the above clinical signs. Subclinical infections may not show the traditional white spots on the skin and only the respiratory clinical signs may be observed.

Causes of White Spot Disease in Saltwater Fish

White spot disease in saltwater fish is caused by the ciliated protozoan parasite, Cryptocaryon irritans. It has an identical life cycle and pathology as its freshwater counterpart, Ichthyophthirius multifiliisCryptocaryon irritans has a complicated life cycle that includes a feeding stage, known as a trophont. This part of the life cycle occurs in the gill tissue, causing damage and decreased respiratory function. It is also the stage seen as the "white spots" on your fishes' skin. At the end of the trophont stage, the mature parasite leaves the host and drops to the substrate where it encysts into a tomont. After internal cell division, the cyst bursts and releases up to 1,000 free-swimming tomites, or theronts. Each one of these tomites attaches onto a fish and develops into a trophont, and after feeding can encyst to produce 1,000 more offspring, illustrating how quickly this parasite replicates. This entire life cycle can be accomplished in six to 11 days, depending on the water temperature.

Diagnosis of White Spot Disease in Saltwater Fish

Although white spot disease is easily visible on the fishes' body, it can be confused with lymphocystis or fin ray fractures. In order to confirm a diagnosis, your veterinarian will perform a skin mucus scrape and gill biopsy. These are best performed on a sedated fish. When examined under a microscope, this will confirm the white spot disease or point towards a different diagnosis.

Treatment of White Spot Disease in Saltwater Fish

When treating white spot disease, it is critical to take into consideration its complicated life cycle. There is no effective treatment for the encysted stage of the life cycle! For a successful treatment, you must perform repeat dosages of medication in order to kill the free-swimming tomite stage. The feeding and encysted stages of the parasite are resistant to medication. The length of your treatment will depend on your water temperature. The warmer the water, the faster the parasite will complete the life cycle. However, do not just increase the temperature to get the treatment over with faster. You may inadvertently stress your fish and compromise their immune system, making it easier for the parasite to penetrate and kill your fish.

Not all over the counter treatments are the same. They may use different active ingredients and have different effects on your tank's inhabitants. One of the most common treatment additives is copper, which is toxic to invertebrates, including crustaceans and corals. If you are able to evacuate the fish from the tank and treat them in a separate hospital tank, the white spot disease life cycle will be broken in the main aquarium since there are no fish to host them. If you are unable to remove the fish and you do have corals, crabs, shrimp or other invertebrates in your aquarium, you will have to choose a treatment that is safe for invertebrates.

Hyposalinity, or decreasing the salinity of the water, does not require any chemical additives, but is not safe for all fish and invertebrates. If you have sensitive fish or corals and are able to remove them, you can treat the delicate fish with another treatment. In order to be effective against saltwater white spot, your salinity will have to be below 16 ppt (16 mg/L) for at least three weeks.

If you are concerned about treating your fish correctly, discuss your treatment options with your veterinarian.

How to Prevent White Spot Disease in Saltwater Fish

The best method of treating white spot disease is to keep it out of your aquarium in the first place. This is accomplished by an effective quarantine protocol in a separate tank with separate filtration equipment. A minimum four-week quarantine period is recommended. If after that the fish are healthy, they can then be transferred into the aquarium. Treating the fish while in a quarantine tank is much easier than treating the main aquarium.