I think you may have a pretty complicated problem. I still think that there is some imbalance to this tank. Even though its been going for 2 months the excessive water changes may not allow it to totally balance out. You say your water parameters are fine, is your ammonia totally zero as wellas your nitrite? How do the fish act before they die? Do they turn dark, do they breathe heavy, sit on the bottom of the aquarium? Are there any other symptoms? I can't really see a stingray harming the Frontosas. What kind of biological filter do you have?

