What could I add to my fish tank?

Well, you can really add any fish, just not a fish a betta would eat. And betta's are supposed to be extremely aggressive, so you should be careful. one thing to not add, a guppy, cause I had a betta too, and I brought a guppy for it to eat, and it tore it to chunks. Other than that, you should be fine, just not any small fish or your betta might eat it