Agreed w/ Naturegirl... I don't mean to come across as rude, but this happens much too often. They may seem fine one day, but the next day you could wake up to find one dead, and/or one injured.
Yes, as NG243 has said, males can be kept in seperate or divided tanks, and female in sororities- 10 gallons and up, 4 or more females (some people say six.)
In the hobby for 9 years.