Penguin Tetra?

[Olympia]

Stress from being in a small group and small tank. They do have a pecking order, like female betta. That is why you need at least 5-6 tetra. They will need a larger tank when they grow up, and a larger school. Otherwise your little guy is going to get bullied to death.

[Bombalurina]

+1 to Olympia. I also strongly advise against using Pimafix in a tank with a betta. Firstly, it is not really useful, being almost entirely eucalypt oil rather than medication, and secondly, it can fatally irritate a betta's labyrinth organ.