From the Super Mario Wiki
The Pipe Room (Japanese: 配管室 Piping Room) is a room in Area Four of Luigi's Mansion.
Once Luigi enters the Pipe Room, a set of green Flying Fish and Red and White Grabbing Ghosts attack. After Luigi captures them, he uses an Ice Elemental Ghost to get to a valve that stops the flow of water. This causes the Key to the Cold Storage to appear.
The Pipe Room contains a crate, a bucket, and a frozen barrel. The river of water is untouchable until Luigi freezes it.
Ghosts in the Pipe Room in the PAL Hidden Mansion