Aqua City is now the first thing visitors see when they enter the park. It covers an area of 20,000 square metres and consists of three major attractions: the Grand Aquarium, the Lagoon and the Sea Life Carousel.
The Grand Aquarium has a viewing dome measuring 5 1/2metres in diameter and a 13m-wide viewing panel. It is home to 5,000 fish and more than 400 marine species.
The aquarium holds 5.24 million litres of water - more than twice that of the old one, which held only 2.2 million litres of water.
It has been designed to look like a sea creature - an egg-shaped structure with strips of fins swirling around the outside.
"It will take visitors to a brand new multi-sensory experience," says David Lai Yiu-nam, curator of aquariums at Ocean Park. "Apart from watching species in our different tanks, there are also three-dimensional video presentations for people to learn about the oceans."
Visitors will see the Scalloped Hammerhead Shark, Manta Ray and the Pacific bluefin tuna swimming together in the huge tank.
"These are rare species one can seldom find in any aquariums in the world," says Lai.
He rescued some tuna fish, with the help of fishermen, that had been destined for local fish markets. "Usually we see only a piece of tuna on a sushi plate; now we can see the whole fish in real form - and alive. We hope by letting people see them, they will care more about them and think about conservation."
Aqua City also features a night-time show for up to 4,200 people called Symbio, presented on a magical 360-degrees water screen in the Lagoon.
The show uses water, fire, special effects and music to tell an ancient story of two powerful dragons. The story symbolises the relationship between humans and nature.
Two educational facilities help visitors learn more about the importance of ocean conservation.
The Ray and Shark Encounters give you a close encounter with the creatures by snorkelling in the water beside them, and the Ocean and Coral Classrooms offer activities to help visitors learn about ocean and coral habitats.
"We hope to bring out the message that humans and oceans are intimately connected," says Suzanne Gendron, executive director of the park's zoological operations and education.
"We should live in harmony and work together to protect a healthy, balanced ecosystem."
Ocean Park has had more than 99.4 million visitors since it opened, and the park hopes the new attractions will help boost visitor numbers.