columbian sharks

Hi has any on had any luck with colombia sharks. I have  a 20 gallon with 3
iradesant sharks 2 plecostimass and 2 red tail sharks. Levels are all good
tank has been established for over three years. I am trying to add a school
of columbia sharks. I added then properly but with in a week two will die.
it has happend twice now any tips would be great. I bought the fish at
walmart. but have had good luck with other fish from there as they actually
have someone working in the fish department that has fish and know what he
is talking about. any way if anyone can help i would apprciated it.