A number of texts from antiquity have contentiously suggested the ancient Romans could transport live fish by sea. For instance, the scientist, Roman officer and historian Pliny the Elder spoke of transport of parrotfish from the Black Sea to the coast of Naples.
[Researchers] estimate an aquarium behind the mast of the ship could have measured about 11.4 feet by 6.5 feet by 3.3 feet (3.5 m by 2 m by 1 m) for a capacity of approximately 250 cubic feet (7 cubic meters). For comparison, an average bathtub has a volume of about 7 cubic feet. If properly maintained, it could help keep at least 440 pounds (200 kg) of live fish such as sea bass or sea bream, they noted.
Cronaca again (who has lots of good stuff lately).