Mating?

[Deadmau5]

Ok, so I've never exactly seen fish mate, other than Betta fish. My two swordtails, a male and a female, have been very jumpy all day. He keeps chasing her around, she keeps swimming away but he catches up and nips at her, tries to trap her in corners and get close to her. He has also done it to my platys, but mostly to my female swordtail. His top fin is usually down, but it's straight up in the water now, and not going down. Does his mean they are trying to mate? Well is he trying to? I've always bought my fish already pregnant :L