Belief that owning fish are bad luck?

My boyfriend's family (they are from South America) thinks that having a pet fish in the house are bad luck. He's unsure of me moving my betta fish in. What's the deal with the belief that fish are bad luck? Is it a cultural thing? I know Asians believe that fish are good luck. I'm not superstitious personally, but I've never even heard of this one. Can anyone shed some light on this?