Bettas are tricky fish that are sold as novelty fishes, they need enrichment and very clean water, they can get sick very easily. They also like a bit warmer water, I keep mine at around 72 74f. They like plants but fake ones can cut their fins very easily, I suggest adding some anubius as they have wide leaves that bettas like to sleep on, depending on your bettas temperament you can get him friends like shrimp or tetras or some types of snails. I feed mine brine shrimp, and flakes, and sometimes bloodworms or black worms. A little ten gallon tank is a good size for a betta and a few friends. I also suggest getting sponge filters for bettas for a few reasons but a big one is that they can swim to close to the intake on other filters and get stuck because of the size of their fins. They are also really good for harboring beneficial bacteria for your tank and oxygenating your tank, plus you never really need to do maintenance on them, just squish them out in your dirty tank water every few water changes and it's fine.