Sudden Death - A Mystery

I received four Swordtails via FedEx from a breeder four days ago. Three hifin lyretail females and a hifin male. They all seemed healthy and spry. They have been quite skittish since arriving and still spend most of their time hiding. Today at 5PM, everything looked fine as I fed them. But soon after, the male started sinking like he'd swallowed a lead weight. At times, he'd swim energetically up to the surface, just to sink back down to the bottom. He also seem to have a kink in it's spine and was breathing heavy. By 7PM, he was dead. I did a crude biopsy suspecting he may have swallowed something inappropriate, but that was not the case.

I'm thinking it may be a bacterial infection. I'd greatly appreciate suggestions on treatment, as I'd like to start treatment tomorrow.

Thanks for your response. The fish did not have a kink when he first came in, or when he died. Just in the couple hours he was in misery. In fact, he was quite a spectacular specimen.

I emailed the breeder, who is guessing that he injured his swim bladder by running into something while darting around the tank. All four where sharing a 10G quarantine tank and had grown up in a 30G long tank. That explanation fits with the suddenness of the death, and that the fish was darting around much more has he was getting comfortable in his new surroundings.

I'm still open to other theories.