betta x cardinal tetra

For compatibility of behaviors, cardinal tetras are generally peaceful and typically do better with similar sized tank mates. With a betta, they will probably have better luck with a female betta but shouldn't have much issues with a male if the personality matches. Having a larger group (6+ individuals) will help them focus on each other and ignore picking on other tank mates. Having a larger group also allows for more diversity of a gender ratio which should also help the group focus on each other more as well. For compatibility of parameters, both of these animals are best kept around 78-80F. Both of them can tolerate a range between 5.5-8.0ppm, but it's more so important to keep the tank pH stable at one level than try to fluctuate and stress the fish. For tank size, it's recommended to not go under a 40g breeder to keep these two together for first time keepers.