Black Spot or Tang Disease in Fish

Yellow Surgeonfish
The yellow tang (Zebrasoma flavescens) is a saltwater surgeonfish and is one of the most popular aquarium fish. It is often found with black spots on its body due to parasites. Santiago Urquijo/Getty Images

Black spot "disease" is caused by a parasite that commonly infests Tangs and other Surgeonfishes, but can be contracted by other fish species and mollusks (shellfish) as well. Black spot disease is also referred to as tang disease or black ich, although ich is actually caused by a different type of parasite. The black spots on the fish are tiny Paravortex turbellaria flatworms (Phylum Platyhelminthes) embedded in the skin of the fish. Black spot parasites are much less dangerous and life-threatening, as well as easier to treat, compared to protozoan parasites such as Oodinium, Cryptocaryon (ich disease) and Brooklynella. Nonetheless, it is a problem where fish infected with these parasites need to be treated.

The Life Cycle of These Worms

  • After "hatching out," the juvenile (200 micrometers long by 50 μm wide) Paravortex turbellaria worms can swim freely and exit the substrate seeking out a host fish. Once they find a suitable host fish, the juvenile turbellaria burrow into the epithelium of the fish's skin or gills and begin feeding on the fish's cell fluids.
  • After feeding on a host fish for about six days, the mature turbellaria (now 2500 μm long and 750 μm wide) fall off the fish and descend into the substrate.
  • About five days later, the worm's body ruptures and releases a new population of about 160 young worms, which can immediately swim about and attach to a new host. The cycle starts again but in larger numbers.

Symptoms of Black Spot Disease

Once these worms start feeding on a host fish they acquire melanin pigmentation, which causes the appearance of black spots about the size of a grain of salt on the body and fin membranes. The worms have the ability to freely move about on fish as the spots do not always remain stationary. On light colored fish they are easy to see, whereas on dark colored ones they may go unnoticed at first.


As with other surface parasites, the infested fish will scratch up against objects or the substrate in an attempt to dislodge the parasites, and if the infestation is allowed to progress the fish can become lethargic, lose their appetite and colors, and if the gills are affected rapid respiration develops. Secondary bacterial infections can invade damaged tissue sites. If surface infections develop, treat as with any other bacterial infection.

Treatment

Give all infested fish a freshwater dip, followed by a formalin bath and continue treatment in a quarantine tank. Praziquantel has been used with some success to treat affected fish in quarantine tanks.

Preventing Reinfection

Reinfection will occur no matter how effectively the fish have been treated if these parasitic turbellarian worms are not eradicated from the main aquarium. Unfortunately, because they can survive for several months without a host, this is often not an easy thing to do, but here are some suggestions.

  • Leaving the main aquarium devoid of all fish for several months is a cure that some aquarists recommend, however, this is not always possible or desirable. The theory here is that, without a host fish, the life cycle chain will be broken and the turbellaria will not be able to reproduce and you will just be waiting for the turbellaria to die of old age.
  • Young worms live in the substrate and feed on detritus and organic debris until such time they go in search of a fish host. Siphoning up the excess organic matter that builds up on the bottom of the aquarium can help to control their numbers.
  • For fish-only tanks, or marine aquariums containing no freshwater-sensitive invertebrates, hyposalinity can be applied. When using hyposalinity (osmotic shock), stirring the substrate occasionally will help to release the higher salinity water trapped in the substrate, exposing the flatworms to the lower salinity water in the tank.

Natural Predators of Turbellaria Worms

Turbellaria worms are after all, marine flatworms. There are some fish which will consume them when given the opportunity. These fish include six-line wrasses (Pseudocheilinus  hexataenia), yellow wrasses (Halichoeres chrysus), striped mandarinfish (Synchiropus splendidus), spotted mandarinfish (Synchiropus picturatus) and no doubt, any other Dragonette.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.

Six Line Wrasse (Pseudocheilinus hexataenia)
Six Line Wrasse. David & Micha Sheldon / Getty Images
If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.