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It is often claimed that Siamese fighting fish (Betta splendens) originate from and prefer to live in small puddles of water, and that they will become stressed in a tank larger than a few litres and may even die. This is incorrect.
The natural habitat of Siamese fighting fish is in large, heavily vegetated marshes, rice paddies and slow moving streams of South Asia (in Thailand and Cambodia) which although they are shallow, are deeper and provide a much more varied environment than the majority of fish tanks.
A behaviour that encourages this misconception is the way in which these fish breathe air at the surface. Siamese fighting fish possess a specialised labyrinth organ which allows them to take in surface air, and this supplements oxygen in the water. They cannot survive on surface air alone (they also need oxygen in the water) but it is a necessary form of oxygenation - without surface air oxygen they can drown and die. Surface air breathing assists Siamese fighting fish in surviving in their natural habitats where the water tends to be low in dissolved oxygen due to slow currents and high vegetation densities. It also acts as a survival mechanism if fish become trapped in puddles due to evaporation in the dry season.
The ability to supplement oxygen with surface air is a survival strategy that helps Siamese fighting fish tolerate living in puddles while they attempt to escape to larger areas by using their excellent jumping abilities. Thus, while it is possible for Siamese fighting fish to survive a short while in a small tank, this is not an appropriate environment for permanent housing.
Keeping any fish in a small volume of water is a serious welfare problem for a number of reasons. Firstly, the water will rapidly become toxic as the available oxygen is quickly consumed and ammonia accumulates from the fish's waste. In addition, small tanks cannot hold the correct temperature required for tropical fish, and do not provide sufficient space for exercise or behavioural stimulation, both of which are fundamental in providing a healthy, stress-free environment.
All fish, including Siamese fighting fish, require ample space to swim freely and exercise. As Siamese fighting fish originate from densely vegetated areas, the tank should include several plants (real or artificial) and/or other forms of cover in which to explore, rest and hide if they feel threatened.