Why do fish get bloat? What causes it?

One of my fish hadn't been eating very much in the last couple days, we just got them so wasnt sure what hteir eating patters were. they are baby trophie fish, noticed the bigger of the babies wasn't swimming right and eventually we watched him lay down and die. I called the store and they said they think its bloat. Its to late to save him now, just worried about the other fish. We separated them and gave them fresh water. I am just wondering what could have caused it?