In order to access this secret star, you will need to have three Power Stars in your possession. In the main lobby, look to the right and you will see a door with a “3” on it. Open this door and you should see a painting in front of you, but that’s not the goal. While facing the painting, look up and to the right to see a hole in the wall. Jump up to this hole and go inside it to enter the secret aquarium. First and foremost, the entire place is filled with water, so you will not be able to find any spot to go up for air. That means you will need to collect coins in order to refill your life.
So, your goal in this secret aquarium is to collect a total of eight Red Coins. They are really easy to get, too, as they are all really close to one another. In fact, there are four rings of regular coins that have a Red Coin in the center of them, located at four points. The other four Red Coins are found on the floor, in the corners of the room. Once you collect all eight Red Coins, the Power Star will spawn in the middle of the room.