Capybaras in the aquarium, hm?
That's probably more common than manta rays in aquariums... For example, in the US, there's only one aquarium that keeps a manta ray. For Japan, I can only find articles about an Okinawan aquarium with a manta ray program. The smaller of the two species typically has a wingspan of 3-3.5 m, so they need a lot of space. And both species are in a threatened conservation status.