Difficult to answer without more details.
What kind of tank do you have them in? Was it cycled before the fish were added and how long has it been set up? What, if any, tankmates are there? Were there any symptoms before and after the fish died? How were the new fish introduced? Too quick can send them into shock.
My thoughts is there's something wrong with the tanks water and a full spectrum water test is in order.
What kind of tank do you have them in? Was it cycled before the fish were added and how long has it been set up? What, if any, tankmates are there? Were there any symptoms before and after the fish died? How were the new fish introduced? Too quick can send them into shock.
My thoughts is there's something wrong with the tanks water and a full spectrum water test is in order.