The best size tank for a Betta fish is a 5 gallon one. Multiple factors determine the lifespan of a Betta fish, and tank size is one of them. As many make mistakes in thinking a smaller tank would cut it, that's not the case. Betta fishes require at least a 5-gallon take to move around freely! In a smaller-sized aquarium, Bettas can live around one to maybe two years tops! But, the bigger the tank, the better the home you are offering your fish. Although it may seem huge compared to the fish, it's the least you can do for them. Of course, if you have room for a bigger tank, go for it!