So... what is a good betta fish tank size? I recommend 2.5 gallons and up, and have had a lot of success and had some very happy bettas in 2.5 gallon tanks, though this is the smallest I would recommend, and you must also consider the shape of the tank as well. A tall column shaped 2.5 gallon tank will not have a sufficient amount of surface area, and will not be a good choice. With a 2.5 gallon tank, decorating the aquarium can be a challenge. Betta's love to have good hiding places, but you also must not "crowd" the tank too much either, which can be tough with the small footprint of a 2.5 gallon betta tank.
I find 5 gallon aquariums to be a great size for betta fish. They give you plenty of room to give the betta several good hiding places, plus leave some open room for swimming. It also gives you enough space to work with live plants - which bettas love. In addition, any aquarium under 5 gallons is very difficult to keep at a constant temperature. There are a few aquarium heaters made for smaller tanks, but in my experience they do not work all that well, plus they take up even more valuable space. Make sure the heater you get has a thermostat - many of the models for small aquariums are just "always on" which can lead to overheating quickly.
How big is too big? Most wild betta fish will try to dominate an area 3' x 3', with water up to 12" deep, which is over 67 gallons of volume. Keep in mind, these areas are heavily planted with lots of hiding places. The biggest thing to consider, is height - you don't really want a betta fish tank higher than 18" - 20" so they can get to the surface fairly easily. That means you could have a single (very happy) male betta fish in a heavily planted and decorated 55 gallon aquarium, and it wouldn't be a problem for the fish at all.
Whatever betta fish tank size you choose, remember to consider the weight of a filled aquarium. A single 5 gallon aquarium typically weighs around 62 pounds when filled. Below is a chart of the sizes and average filled weights of the standard aquariums most suitable for betta fish. There are also a lot of "specialty" aquariums, especially in the 2.5 - 10 gallon range. These often have unique shapes, such as a bowed or wavy front, though their filled weights should be similar to the ones listed below of the size water volume.
|Tank Size||L x W x H||Filled Weight|
|2.5 gallon||12" x 6" x 8"||27 lbs|
|5 gallon||16" x 8" x 10"||62 lbs|
|10 gallon||20" x 10" x 12"||111 lbs|
|15 gallon||24" x 12" x 12"||170 lbs|
|20 gallon High||24" x 12" x 16"||225 lbs|
|20 gallon Long||30" x 12" x 12"||225 lbs|
|25 gallon||24" x 12" x 20"||282 lbs|
|29 gallon||30" x 12" x 18"||330 lbs|
|30 gallon Breeder||36" x 18" x 12"||348 lbs|
|40 gallon Breeder||36" x 18" x 16"||458 lbs|
|40 gallon Long||48" x 12" x 16"||455 lbs|
|50 gallon||36" x 18" x 19"||600 lbs|
|55 gallon||48" x 13" x 21"||625 lbs|
Another thing to remember - is that betta fish are jumpers! That means you need an aquarium with a secured cover or lid, otherwise you risk having your finned friend hop out onto the ground. With ample room to swin, and plenty of places to hide, your betta fish should be one very happy little guy!