BACKGROUND: Goldfish (Carassius auratus), a carp fish of Cyprinidae family, is one of the most popular aquarium fishes in Iran and also in the world. Ornamental fish trade, especially Goldfish, is aged over a hundred years. If we believe that any aquatic species or country has its own specific and unknown fauna of pathogens, the risks of transmission of these pathogens by importing ornamental fishes (whether at national, provincial or municipal level) will become more obvious. However, few researches have been done about parasitic infections in ornamental fishes. Objectives: Identification and investigation of fish pathogens, especially parasites, prior to transportation to new place is considerably important in the preventing and restricting the transmission of these pathogens.                      Methods: In this study, skin and gills of 100 goldfish were examined by wet mount in order to investigate monogenean parasites. Results: Nine species of parasites from 3 families (Dactylogyridae, Gyrodactylidae and Ancylodiscoididae) were isolated and identified in fishes, including 4 species of Gyrodactylus (Gyrodactylus kobayashii, Gyrodactylus Gurleyi, Gyrodactylus longoacuminatus and Gyrodactylus sp.), 4 species of Dactylogyrus (Dactylogyrus vastator, Dactylogyrus formosus, Dactylogyrus baeri and Dactylogyrus sp.) and 1 species of Ancylodiscoides. Gyrodactylus kobayashii (10%) were the most common parasites found, followed by Dactylogyrus vastator (9%). ConclusionS: Although these fishes are quite resistant against such pathogens, transporting them to a new aquatic ecosystem could transfer these parasites and cause infestation of healthy fishes, which brings about financial losses and casualties.

The present study was carried out to find out the histopathological alterations in gill, liver and kidney of Goldfish (Carassius auratus) under sublethal concentrations of Zinc. Fishes were exposed to sub-lethal concentrations of 0.572, 1.145, 2.289, 4.578 and 9.156 mg.l -1 for 28 days which are 1/10 of their 96hr LC. The histological samples of gill, liver and kidney were collected from 14 th, 50th  and 28th day and stained by haematoxylin and eosin, observed under trinocular microscope. Vacuolation, hyperplasia, epithelial lifting and lamellar fusion, shrinkage of blood vessels, secondary lamellar damage, breakdown of epithelial cells, blood congestion and necrosis were observed in gill. Rupture of nucleus, congested blood vessels, hemorrhage in liver sinuses, increased size of kupffer cell, increased pycnotic nucleus, hypertrophied hepatocytes, vacuolation, cellular necrosis, rupture of hepatocytes were observed in liver of zinc treated fish. Shrunken glomerulus, vacuolation, blood congestion, increased in space of periglomerular, increased in space of peritubular, melanomacrophages, loss of cytoplasm, fat deposition, degenerated tubules and degenerated glomerulus were seen in kidney. The above results showed that sub-lethal toxicity of Zinc alter the internal organs of goldfish at lower concentration.