Kaiyūkan is among Japan's best aquariums. An 800m-plus walkway winds past displays of sea life from around the Pacific 'ring of fire': Antarctic penguins, coral-reef butterflyfish, unreasonably cute Arctic otters, Monterey Bay seals and unearthly jellyfish. Most impressive is the ginormous central tank, housing a whale shark, manta and thousands of other fish. Note there are also captive dolphins here, which some visitors may not appreciate; there is growing evidence that keeping cetaceans in captivity is harmful for the animals.
There are good English descriptions, but the audio guide (¥500) gives more detail. Expect lots of families and school groups.
The Osaka Kaiyu Ticket (www.kaiyukan.com/language/eng/kaiyu.html; adult/child ¥2550/1300) combines entry to the aquarium, with one-day unlimited transport on city subways and discounted admission to other sites. Purchase at any subway station. Check the schedule online, as the aquarium closes a few times a year.