Kaiy奴kan 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 rays 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 (楼500) gives more detail. Expect lots of families and school groups.
The Osaka Kaiyu Ticket (www.kaiyukan.com/language/eng/kaiyu.html; adult/child 楼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 for a couple of days in January and February.