I don't know if this will help or not. 1. Sometimes fish are sick when you buy them but it takes a while to show up - up to 4 weeks. 2. The other fish may have been picking on him. 3. He may have died due to stress - this also can cause white spot. Sometimes just shifting tanks can do it. Do the fish still have white spots on them and if so more or less? The only thing I can really say is that guppies are pretty tough. If only one has died its probably nothing to worry about, but if anymore get sick/die, stop eating, swim funny/look strange, let us know.