Got check the fins of the dead fish? Could be due to stress from transport and possible harrasement from the dominant damsel that already "claimed" the tank. Either way it could be due to sudden rise in pollutants in the tank leading to a spike in ammonia nitrite or nitrate. Prolly should check for all 3 when introducing new fishes specially if its 3 at one go and your system only used to one fish. All the best!!!