Although bettas are frequently referred to as "fighting fish," they are docile with some fish and can make excellent tank mates. Male bettas should never be housed with other male bettas. Bettas' personalities vary, and if yours is overly aggressive, avoid placing him with any other fish.
Named for their transparent, ghost-like appearance, ghost shrimp tend to hover at the bottom of the tank and scavenge for debris. Because they are small, they're unlikely to harass your betta, and their transparent skin makes bettas unlikely to notice them. They also will help keep the tank clear of debris. Another excellent debris-eating fish is the Otocinclus catfish, a small, docile, brownish catfish that will spend most of his time at the bottom of the aquarium.
Like catfish, many species of snail eat tank debris and algae, acting as an additional layer of filtration in the betta's tank. They move slowly and your betta is unlikely to notice them, though there is a chance he will nip their long, wavy antennae. Keep in mind also that some species of snail will devour your aquarium plants. Good choices include zebra snails and apple snails.
African Dwarf Frogs
African dwarf frogs are small, speckled water frogs that are relatively easy to care for. They move quickly, making them unlikely prey for bettas. However, some frogs behave aggressively and will harass your betta. Consequently, it's important to carefully observe the tank for the first few days. Bettas and dwarf frogs are more likely to successfully live together in large tanks—30 gallons or more.
Neon tetras are small schooling fish with bright stripes. They tend to congregate in the middle of the tank, while bettas prefer the top. This makes them less likely to interfere with the betta's territory, and bettas generally don't display aggression toward these fish. Because tetras are schooling fish, you must keep them in groups of six or more, which means you need a relatively large tank.
Fish to Avoid
Some fish should never be placed with bettas. These include crabs, which may pinch the fins of bettas or even kill them, and guppies, which tend to harass bettas until the bettas eat them. Aggressive fish such as cichlids should also not be housed with bettas, and water turtles are not good betta tank mates.