You don't mention what kind of killifish- there are LOTs of species, and they spawn in different ways. Some are plant spawners, others substrate spawners who need to spawn on peat that is then lifted out and kept damp for months, then reactivated for the eggs to hatch.
You could try the golden barbs; at least the tank is the right size. No idea how hard they are to breed, but might be worth a try.