When I visited the Shark Reef Aquarium I decided to pay the extra $5.00 and go to the new exhibit Sea Monsters Revealed as well. The exhibit is separate from the Shark Reef, but located right around the corner. You buy tickets at the same location.
The Sea Monsters Revealed exhibit is smaller in size, but there is a lot of information and things to look at while inside. I think I was in there for about 20 minutes or so, but I didn't read everything so I'm sure you could be in there for almost twice as long if you did. There are a lot of interesting facts about creatures you can find in the deepest sea depths, and it only gets better as you walk through.
If you have kids, definitely stick to just the Shark Reef and skip this exhibit. I would say that this exhibit is best for kids ages 12 and up, the smaller ones would be bored and not be able to read or understand the exhibit.
All the animals in the exhibit are preserved using a special technique (like the Bodies Exhibit at the Luxor) that preserves the specimens and they are able to last without decay. I especially liked seeing the large sharks that were preserved, I can't imagine how much time and care went into making sure they were preserved correctly!
I definitely recommend stopping by when at the Mandalay Bay in Las Vegas. I can't think of an exhibit like this with animals anywhere else!