How does Marcellus get out of his enclosure, and why does he want to get out from time to time?

Marcellus gets out of his enclosure to find better things to eat than what they feed him. He can stay out of the tank for 18 minutes. At the top of his enclosure is a hole in the glass where the pump comes in. There is a gap between the pump housing and the glass. He can fit a tip of a tentacle through and unscrew the housing. The pump floats into his tank, exposing a gap. The gap is about the width of two or three human fingers. He can shape his body to pass through the hole.