The cultivation and propagation of ornamental fishes have been increasing in the last few years in Iran. GloFish is a registered and trademarked brand of genetically engineered fluorescent fish. GloFish® fluorescent fish are available in a variety of tropical fish species. In an ornamental fish farm of GloFish tetra (Gymnocorymbus ternetzi), mortality of 20 to 30 per day was reported in juveniles and 4 to 6 per day in adult fish. Several fish were sent for sampling while they had been kept in their aquarium water. Wet smears were taken from the skin and gills. After the autopsy, the internal organs were examined for parasitic infestation under a light microscope. No internal parasites were observed. External parasites, including Dactylogyrus sp. in the gills and Tetrahymena sp. in the skin were detected. According to references, treatment with salt bath and formalin 37% were performed. A reduction in mortality was observed during the treatment period. After two weeks, re-sampling was performed and the samples were negative for Tetrahymena sp. and Dactylogyrus sp.  As a result, Salt baths and formalin were effective for the treatment of Tetrahymena sp. and Dactylogyrus sp. in GloFish tetra.