Betta smaragdina, commonly known as the emerald green betta, blue betta or Mekong fighting fish (Thai: ปลากัดเขียว or ปลากัดอีสาน) is a species of gourami native to Southeast Asia. The species gets its green and blue colors due to refraction and interference of light that results from hexagonal crystals that are less than 0.5 micrometres. It is found in the aquarium trade.

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