Despite even the best husbandry, sometimes fish just die. I guess 8 fish in 3 years isn't terrible without a lot of the methods we would preach (qt, carbon reactor, ato, researching stock lists).
As for the corals, that wouldn't be privy to fish diseases, what kind of corals have you tried versus what kind of light are you running? Do you have your heater on a temp controller?
If you wish for good advice you should start keeping a log of your levels, plus list your tank down to the specific equipment you keep, the fish you kept before they died, filtration, maintenance schedule, etc etc.
Basically there isn't enough info in this post to give you any answers or any advice.
Seek the help of your local LFS, if you trust their expertise.