yes, a betta can live fine in a 2.5 gallon, but no less IMO. make sure it's heated, filtered, and lighted with a flourescent bulb. and scarare.... get an azoo palm filter, tera whisper mini internal filter, or a red sae mini filter. for heaters, get a small one about 20-25 watts. and bettas love larger tanks! I used to keep my betta in a bowl (B/C I had no other recourse at the moment.) and he looked so sad... but when I put him in his 10g/ he went nuts with happiness. it's like he never had so much freedom in his life... I felt like singing "born free".