5 gallon will be perfect. I would suggest getting the tank online at Amazon or ebay, they are very pricey in store.you can get the fish, decor and filter from this store. You can save yourself about 30 bucks buying the tank online. However, great selection of fresh and salt water fish they have many different colored beta fish, and a great array of decor. Also, the staff is very helpful and knowledgeable. They will be able to answer any questions you have while you are there. Also they have pamphlets you can take home that will teach you what you need to know about your new little friends.
Betta fish are not technically solitary fish they just do that because of how aggressive they can be. A five gallon tank is perfect size for a small sorority of females. I actually have mine in the Fluval Spec V which is a great five gallon starting kit. Roughly about $80 at this store which is where mine came from.
5 gal is a good size for a betta :) thank you for not putting it in a bowl! Your betta will be much happier and healthier! Any rectangular 5 gallon tank will be fine, just make sure you get a good filter. Personally i like the 50g Aquaclear; it has a flow adjuster so you can make sure your betta is comfortable. Look into cycling a tank and the nitrogen cycle. This is essential for keeping the water safe. Not all bettas actually prefer to be alone, though it's a popular myth! Some really enjoy being in a community tank; just be careful with other fish, especially colorful ones. You can also look into keeping female bettas together (they often call this a sorority tank). If you're keeping females or a shorter finned male, make sure you pick up a lid; they're talented jumpers!
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