Black Spot / Tang Disease (Diplopstomiasis)

Black Spot, or more commonly known as Black Ich and the Tang Disease (due to how common it is found to be infecting Tangs that are caught in the wild) is an infestation of tiny parasite larva Phylum Platyhelminthes. These parasites can not only infect fish that have lived inside of ponds and rivers, but they can also infect snails and mammals alike. Due to the nature of these parasites, they cause very little harm to the fish host as the fish is only the first few stages of the life cycle that these parasites attempt to make.


The major symptom of this disease will include the fish having black spots all over its body that are about the size of salt. Sadly, due to these parasites living on the surface of the skin, they can move around the fish's body at will and mostly go unnoticed for weeks. Another common problem in identifying these parasites is that if the fish is of darker body color, it can be harder to see the black spots when compared to being on a fish that is of lighter body color. As the disease progresses the fish might start to try to rub against decorations, the substrate, and aquarium or pond equipment that is inside of the fish's location.


To understand how to treat black spot completely, we must first understand how the parasites live and infect fish that they come into contact with. These parasites normally start their lifespan growing up inside of the substrate of many different lakes and rivers. As they are free-swimming, they tend to try to find a host fish to infect so that they move onto their next stage in their lifespan. Since the fish are only a means of transportation, these parasites do little to no harm to the fish host but can cause secondary bacterial infections which can kill the fish. When attached to the fish host, they will feed off of its blood for about a week until they reach a certain size.

Once this sized has been reached, they normally fall off and will descend back into the substrate of wherever the fish is located at now. In roughly a week, the main parasite will rupture its body and release hundreds of new parasites to repeat this process. In nature, these parasites will only move onto the next stage of their lifespan if they can be eaten by a mammal or bird from an infected fish.

Treatment and Medication

Since we now understand the life cycle of black spot, we can see that our treatment has some guidelines that it must follow to kill these parasites at their weakest point. The most common treatments to remove black spot parasites from infected fish are to use formalin, malachite green, or praziquantel inside of the infected fish's water column. When using any medicine, it is recommended to remove any carbon from the filter media and to also turn off any UV sterilizers or protein skimmers during usage. It is recommended to remove all decorations in the tank before using malachite green as it can stain them a slight shade of green throughout the treatment and if you have any sensitive species within your aquarium please use these medicines at roughly half strength. We recommend following the product's instructions on the bottle when using either of these products.

In many cases even after the treatment of the infected fish, the parasites may still live inside of the substrate layer of the tank. To make sure that there aren't any parasites inside of the substrate, we highly recommend that you wash the substrate out thoroughly with some of the medicine listed above. As a last resort, some have made sure to leave the aquarium empty of all living matter for several months to starve the parasites from having any potential host. Others have emptied their tanks and let them dry for a period of a few weeks before refilling them back up.


Sadly, once a fish is infected with this disease the chances of reinfection from the same water supply are extremely high if the treatments are not done correctly. These parasites can live several months without the need of any host, and due to them also hiding in the substrate greater care is required to make sure that they are all fully removed. Without proper care and treatment, those parasites that may have been living in the substrate can come out and reinfect the whole fish population multiple times.

Due to the way that this disease spreads and affects the inhabitants, the best prevention methods are to always make sure that you quarantine anything that you place inside of your water (this includes any plants, inhabitants, decorations, and more). Thus, when you do let them adjust to your water and you truly see how they are acting, eating and if they have any symptoms or signs, we will either start to get a better understanding if they are infected or learn about what possible other diseases they might have.