Fish diseases and parasites in the context of Ornamental fish

Isopoda is an order of crustaceans. Members of this group are collectively called isopods and include both aquatic species such as gribbles and terrestrial species such as woodlice. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed limbs on the thorax, and five pairs of branching appendages on the abdomen that are used in respiration. Females brood their young in a pouch under their thorax called the marsupium.

Isopods have various feeding methods: some are scavengers and detritivores, eating dead or decaying plant and animal matter; others are grazers or filter feeders, a few are predators, and some are internal or external parasites, mostly of fish. Aquatic species are mostly benthic, living on the bottom of water bodies, but some taxa can swim for short distances. Terrestrial forms move around by crawling and tend to be found in cool, moist places. Some species are able to roll themselves into a ball (known as volvation) as a defense mechanism or to conserve moisture like species in the family Armadillidiidae, commonly called the pillbugs.

The term fish kill, also known as fish die-off, refers to a localized mass die-off of fish populations in a body of water, which may also be associated with more generalized mortality of aquatic life. The most common cause is anoxia in the water, which in turn may be due to factors such as drought, harmful algal bloom, overpopulation, or a sustained increase in water temperature. Infectious diseases and parasites can also lead to fish kill. Toxicity is a real but far less common cause of fish kill, and is often associated with man-made water pollution.

Fish kills are often the first visible signs of environmental stress and are usually investigated as a matter of urgency by environmental agencies to determine the cause of the kill. Many fish species have a relatively low tolerance of variations in environmental conditions and their death is often a potent indicator of problems in their environment that may be affecting other animals and plants and may have a direct impact on other uses of the water such as for drinking water production. Pollution events may affect fish species and fish age classes in different ways. If it is a cold-related fish kill, juvenile fish or species that are not cold-tolerant may be selectively affected. If toxicity is the cause, species are more generally affected and the event may include amphibians and shellfish as well. A reduction in dissolved oxygen may affect larger specimens more than smaller fish as these may be able to access oxygen richer water at the surface, at least for a short time.

The slime coat (also fish slime, mucus layer or slime layer) is the coating of mucus covering the body of all fish. An important part of fish anatomy, it serves many functions, depending on species, ranging from locomotion, care and feeding of offspring, to resistance against diseases and parasites.

The mucin making up the slime coat is secreted by goblet cells in the fish's epidermis. The slime contains a variety of antimicrobial peptides and other antimicrobial components such as lysozyme and C-reactive protein. It contains mycosporine-like amino acids to protect from ultraviolet radiation.