Abstract
This study aimed to identify pathological agent and determine the impact of water quality during climate transitions on mass mortalities of neon tetra (Paracheirodon innesi) in the West Java region of Indonesia. Ten thousand fishes reported died each day during the seasonal. Samples of 10 fish/tank were taken and tested for parasites, fungal, and bacteria. The investigations tests showed that the primary disease was caused by parasite from protozoa Pleistophora spp. (Microsporidia). It was confirmed by taxonomic descriptions and histopathological diagnosis. This parasite causes histozoic infection. Fungus was not detected, and bacteria were identified as Aeromonas sp. and Pseudomonas sp.. The water temperature fluctuation was recorded higher than 7°C between daylight and night. Mortality was recorded in every size of fish. The clinical signs of fish on the outbreak were pale body, discoloration of the lateral line, the appearance of white patches under the skin, lethargy, swimming disorder, and losing weight. The recommendations due to the outbreaks were total eradication for infected fish populations, separating newly arrived fish (quarantine) at least 2 weeks, maintaining water quality in the optimal range, and proper cleaning (sanitizing) must be performed.