Many people tend to get just male guppies for their tanks for the color and plus they do not want any females due to breeding. Well part of the problems with that is what Oriongirl has stated they are hyper lil fish. They want to breed all the time and will bully your other fish be it a male or female guppy. The only way to prevent this is to keep trios, 1 male: 2 females, 1:2. The reason why your fish died? well the problem with many live bearers is that they are over produced, kept in poor water conditions and then sold off to be placed in crowded store tanks and again kept in poor water conditions. This causes alot of stress on these guys so when added to a new tank, many perished. Now many will say you need to add salt to your tank for any live bearers. Well do not belive them. Salt causes more problems for live bearers in the long run.