Black spot disease can affect many species of freshwater fish primarily affecting surgeon fishes and tangs. It also inhabits molluscs (shellfish). Black spot is caused by digenean trematodes (flukes) in the families Diplostomatidae and Heterophyidae.  The cercarial forms of the trematodes enter the fish through the skin. Encyst is the next stage where development happens and the parasite enters metacercariae. The fish immune system surrounds the cyst with black pigmented melanin in response to the foreign organism, thus creating the identifying black spot of its name sake. 

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Carp black spot is transferred from fish to fish by parasites. Transferal can occur through faecal matter from birds, small mammals and other wildlife that have eaten infected carriers, but also will occur through transferal of invertebrates and parasites on equipment used in or on the water. Treatment of this disease is simple with regards to ornamental ponds and tanks but has a bigger impact once it has penetrated a water course. The parasite that transfer the disease has a complex life cycle using multiple host to mature before reaching its ultimate hosts, the fish. A mild antibiotic can be used in ornamental ponds and tanks but may not be effective for larger water bodies either through costing or the adverse effects it may have on the rest of the ecosystem.