What is the most difficult creature to keep alive in a salt-water aquarium?

Question:

Years ago I would have said plants and/or filter feeders. But I see some people, and public aquariums, are now having better success with keeping both alive! Now I would guess sand dollars (never see them in any salt water aquarium; big or small) and the great white shark. Read that great white sharks give off electricity around their body to "read" their environment so no matter how big or round the they tank they are in, the metal in the walls messes up their 'readings.'

Answers:

They get up to 6 feet and need fresh water lol

The largest aquarium in the world in Japan has whale sharks. That sounds like a big task.

I think all animals that need oxygen.

some very hard species are Fiddler and Round stingrays. Very hard to care for in a home aquarium.

it might be the sting raya blue whale