I really wanted to goto the Aqua Planet Aquarium but this had the longest line in the Expo starting from 8am and stretching thru the expo site. 
 
Basically you have the choice, either you will do the aquarium, or you will do other stuff. I opted to goto the company pavilions as I'd get to see more and try to come back to the aquarium later, but I ran out of time.  

The brochure reads

The largest in Korea accommodating globally rare species, another place for a great learning experience

At Korea's largest aquarium in Yeosu Expo, take a closer look at a total of 280 species and some 30,000 sea creatures. Among them are endangered species from around the globe such as Beluga Whales and Baikal Seals from Russia

Approx Tour Time : 60 Minutes

Here is a good album from Korea Tourism Organization posted on facebook
https://www.facebook.com/#!/media/set/?set=a.10150962905352492.452962.340785907491&type=1

I would like to go back to see this if possible

Good news is this will be a permanent exhibit and will remain after the expo ends