From Camille Saint-Saens, The Carinval of the Animals, "The Aquarium."
From In Classical Mood listener's guide:
Rippling runs and arpeggios(chords where notes are played one after the other) on the piano conjure up an aquatic scene in this "musical" aquarium. The sinuous melody of the violins and flute suggests fishes gliding through the water, while the subtle variations in the piece all help to paint a magical picture of underwater life.
Many people love the soothing sounds and images of water. It is a kind of therapy that calms most people. Probably the reason why so many dentist and doctors have aquariums in their waiting rooms.