A male betta can never share a tank with another male. RSPCA Australia names snails and large shrimp as suitable companions.
Tank mate
What the sources say
Why tank mates are tricky
Male bettas aren’t aggressive towards other kinds of fish, OATA says, but other fish often nip their fins hard, which can cause stress and even death. So OATA keeps males alone or with very placid tank mates.
Room for company
RSPCA Australia gives at least 40 liters (10.6 gallons) for one betta and a shoal of small community fish, double the 20 liters it calls ideal for a betta alone. RSPCA adds that bigger is better: larger tanks hold steadier temperatures and water conditions.
Common questions
Can I put a mirror in a betta tank?
No. RSPCA Australia says mirrors should never be used in a Siamese fighting fish's tank.
Can two female bettas live together?
Not by either source: OATA keeps females in groups of at least four, in at least 60 liters, and a University of Illinois vet says a betta shouldn't share a tank with any other betta.
What can share a betta’s tank comes from OATA, RSPCA Australia and a University of Illinois veterinary column. Checked against published data from OATA, NIST, RSPCA, RSPCA Australia and University of Illinois.
RSPCA, Ornamental fish: Environment (“Choosing an aquarium”)
OATA (Ornamental Aquatic Trade Association), How to care for bettas, September 2022 (PDF)
OATA, How to care for freshwater shrimp and snails, September 2022 (PDF)
RSPCA Australia knowledgebase, How should I care for my Siamese fighting fish?
University of Illinois College of Veterinary Medicine, “Bettas Need More Than Bowls” (pet health column, 2019)
General care guidance from animal welfare charities, vets and makers' published sizes. It isn't veterinary advice: if your pet seems unwell or has eaten something it shouldn't, call your vet.