Not sure why they are dying but can assure you that its not beacuse of a male. you can have as many males as you want in a roup the never harras to the point where it would kill. Before I got ride of my group I have 7 males with one female in a standerd 55 gallon tank, all of who were around 5 inches. Best of luck with your remaning 3. Also if you ever want to get more email me, I get them really cheap
Jon-e
Jon-e