all are males the strongest one will change over , it would have been best to add 2 or 3 at the same time (a pair and a spare) once one is in tank it will bully any other almost to death,how ever it can be done.useing tank diverers,or a trap with the lid almost closed so the smaller one can hide , it will swim in main tank and just go their for safty after a week or two the other one will get along. it worked for me .