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Betta fish - just make sure to give them a few gallons of water in which to swim, a filter for their poo and a heater in their tank to keep them comfortable year round.
It's mostly only the males that don't get along with each other. Given enough space, females can be peacefully kept together in sorority tanks. Similarly, a trio or harem (i.e. one male with multiple females) is typically safe as well and the fish get along just fine, given a large enough tank and appropriate stocking.
Also, most reputable breeders and sources of information will tell you that 5 gallons / 19 liters is the minimum suggested tank size for a happy and healthy fish in optimal conditions. While they can certainly survive in much smaller bodies of water, it's not ideal and in some cases it's actually harmful.