I can make a list but i see where youre going with the question.
Some fish and inverts are actually better off in the sea and not in an aquarium. There's the copperband butterfly fish, any dragonet, any fish that relies on food that is not the norm will be difficult even impossible.
Your'e just starting out so I wouldn't get too crazy with getting a challenging fish. two reasons, first it's not nice to the fish and second they can cost alot of money.
How big of a tank do you have? Some of the more difficult fish require lots and lots of space.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.