Why do my sharks keep dying?

I have a fish tank with a Pakistani loach, discus, yellow severum cochlid and opaline gourami in. At first, in with them was a rainbow shark, which didn't seem very active and died about a week later... Not long after we bought an albino rainbow shark which seemed very well and active, but about two weeks later (today) just died. All of the other fish are in good health, so why is it just the sharks keep dying?