Every Lion Fish Dies?

Okay..so EVERY Lion Fish that my mom &I have bought over the years..will suddenly stop eating &just die. We have a HUGE 500 gallon tank (I think that's the size..) that we'll put one &a few clown fish (2 max) &possibly a star fish. We ALWAYS have enough room for them to swim around..&NEVER over crowd the tank. We was wondering (since our Leo #15 finally died today) why do Lion Fish (after a year or a lil' more then a year) stop out of the blue &will refuse to eat! Even after we take the other fish out..&he's the only one in there...why will he NOT eat?? &No he's NOT ill..we've took care of salt water fish for years &the Lion Fish is our hardest to maintain! Thanks!