Well - I would think the Biggest Monster Saltwater fish that is available for a Private Aquarist - is the Nurse Shark (Ginglymostoma cirratum). They can get to be 10-12' in length, and weight more than 330 lbs. And yes -it is possible for a private aquarist to keep a full grown adult - so long as you have a very large swimming pool like shark lagoon to keep it in.
For a Public Aquarium - The Biggest Monster Saltwater Fish is without a doubt got to be a Whale Shark - which is the World's Largest Fish.
For a Public Aquarium - The Biggest Monster Saltwater Fish is without a doubt got to be a Whale Shark - which is the World's Largest Fish.
