I always recommend a minimum of 5 gallons, and there are a few good reasons for this. Firstly, bettas require heat and filtration just like any other tropical fish. In tanks smaller than 5 gallons, it is tough to find heaters and filters that do the job. Many very small tanks on the market come with weak filters that make things worse instead of better, and no options for heat.
The second reason is that larger bodies of water pollute slower than smaller. One or two-gallon tanks are very hard to keep clean, especially with the lack of filtration options. Ideally, I’d love to see bettas in 10-gallon tanks, but I know that’s not reasonable for many people. Five gallons is a compromise between having a small setup that fits on a desk or shelf and having a large enough volume of water to keep water conditions healthy for your fish.
Finally, there are a lot of myths surrounding betta fish that simply make no sense, such as they prefer very small spaces or do fine in dirty water. This is how bettas end up in plant vases and tiny cubes. No living thing thrives in less-than-ideal situations, so why people think this is appropriate for bettas is mind-boggling. A five-gallon tank gives your fish room to live in a clean and healthy environment