I have been to the Osaka Aquarium Kaiyukan twice. It's always so amazing. The layout of the aquarium makes a lot of bottlenecking, so it is really depends in how into fish and things like that you are. It's basically a big snake path that wraps around the huge central tank and a lot of people pe up as soon as they see the big tank. I been there during new year time a peak season for them to visit here, so it's super crowded. My advice here is if you want to visit Osaka Aquarium Kaiyukan, maybe u can visit during the non peak season.