Most important rule: never two males together
The name says it all: two betta males in the same tank will fight to the death. This isn't an exception, it's the rule — always keep males singly.
A lid is mandatory
Bettas have a labyrinth organ, which lets them additionally breathe air at the water surface. For this they need warm, humid air directly above the water — a well-fitting lid is therefore essential, and also prevents them from jumping out of the tank.
Water parameters and flow
Bettas come from calm, often still waters — strong current stresses them and makes it harder for them to control their long fins. A filter with a gentle, reduced outflow is therefore recommended.
Tank mates
Tank mates for bettas are generally challenging and should only be attempted carefully. At most, calm bottom dwellers like small catfish or snails are suitable. Unsuitable are other long-finned or brightly colored fish that might be perceived as rivals, as well as fin nippers.
Common mistakes
The most common beginner mistakes: keeping two males together, water that's too cold, too strong a current, and no lid — bettas can jump surprisingly high.