Two male Siamese fighting fish will fight to protect territory and should never be placed in the same tank. Females will often be more peaceful together and with other peaceful fish species, however, they have the potential to be aggressive on occasion. If aggression becomes an issue in such a situation, a backup tank is advisable in case separation is needed.
Male and female Siamese fighting fish should not be housed together unless it is for the purpose of breeding. They are likely to become aggressive with each other either before or after breeding. Breeding Siamese fighting fish requires a lot of time, experience and resources as many tanks are needed for the fry (which will eventually fight).
Attempting to house male Siamese fighting fish with other types of fish is not recommended for beginner fish keepers, though aquatic snails or large shrimps (smaller shrimps may be eaten) can be suitable companions. If a community tank is desired, Siamese fighting fish can sometimes be kept with smaller peaceful fish such as corydora or otoclinus catfish and small non-nippy tetras (tetras are best kept in a shoal of five or more of the same species). Widows, Surpae and Cochus Blue tetras are not recommended.
An understanding of the behaviours of other fish species and a larger tank is also required. You will need a tank with a capacity of 40 litres for one Siamese fighting fish and a school of small community fish, with an extra 20 litres recommended for a large shoal or each additional small shoal of another species. Increased plant cover is also advised to reduce stress by providing separate territories and hiding places. A backup tank should be available in case the Siamese fighting fish becomes aggressive or the additional fish begin to nip or harass the Siamese fighting fish, which can often occur with male fish that possess long finnage. Community fish which swim at a different strata to the Siamese fighting fish (such as the mid to lower tank levels) are preferred as Siamese fighting fish often claim the upper sections of the tank.