If you're looking to breed fish, do more research on the species that interest you and properly house/care for them.
If you're looking to put something in your tank because it fits on your desk, I'd suggest shrimp. I personally wouldn't put them in something that small, but it's doable. I don't recommend putting fish in there. You may be able to keep them for a bit, but stunting, death, disease, etc. are not things you want to invite
