May Sally rest in peace

I had five goldfish originally, but then...tragedy struck.

My first goldfish started to get problems with its swim bladder; it was swimming oddly and sometimes going upside-down and not managing to eat a lot. We got it some treatment but it was no use; in about a month it was floating at the top, dead. I remember that good ol' fish, instead of the two sections of tail most fish have, it had three. It probably had something wrong with it from the start.

And two more cassualities followed. Unlike my first goldfish, the deaths weren't led up to and they were basically unexplained. It hadn't been long since we'd changed the water, we'd been feeding them a while-but they were just at the top, dead.

And I can only hope the two goldfish I have left (and they're not orange: they're actually gold, I originally had two orangefish and three goldfish) don't suffer a vicious fate. We need to change the water very soon, maybe I should do it tonight since we're going away for a week.

Anyway, I want to know why fish randomly die like that. Though obviously my three-tailed little fish had problems we could spot.