The aquarium after closing

The last visitor left at six, pulling a child behind her by the sleeve. Oliver watched the glass doors meet and waited for the cleaner to finish checking the toilets. He was supposed to count the chairs in the education room before leaving, but the corridor to it passed the largest tank. During the day, people stood three deep in front of the glass. He had worked at the aquarium for a fortnight and still had not seen the whole window without a row of heads beneath it. Now the corridor was empty.

Blue light lay across the floor like a pool that had forgotten its edges. Beyond the glass, a ray climbed through the water with the slow folding movement of a hand opening in sleep. Small silver fish turned together, and a bright seam travelled through the shoal before disappearing. Behind them, the tank deepened into a darkness that seemed larger than the building. Oliver heard the soft machinery of the pumps, steady as breathing. A shark passed close to the window without a sound, its pale underside briefly erasing the shapes behind it. Then the blue space opened again.

Oliver put the clipboard on the floor. He could see the faint reflection of his white shirt over the rocks, making it appear that somebody stood inside the tank. When he moved, the figure moved with him. A child had asked that afternoon whether fish became lonely. Oliver had directed her to a keeper because he did not know the answer. Alone now, he found the question more difficult than it had sounded. The animals crossed and recrossed the window, each following a route whose purpose he could not identify.

The cleaner's trolley appeared at the end of the corridor. She stopped beside him and looked into the tank. Oliver asked whether she ever got tired of it. She said that she mostly noticed what people had left on the floor. Then she pointed towards a small fish near the bottom. It always waited there at this time, she said, although she did not know why. They watched it hold its position in the current. After a minute she continued towards the entrance, and the wheels of the trolley clicked across the joins in the tiles.

In the education room, Oliver counted twenty-three chairs and one broken stool. He wrote the numbers carefully, then returned by the same corridor. The small fish was no longer in its place. He searched the lower part of the tank before reminding himself that it could be almost anywhere. At the staff door he switched off the corridor lights, leaving the blue window visible behind him. He would ask a keeper about the fish tomorrow. For once, having a question seemed more useful than trying to imagine an answer.

Read the second paragraph, from ‘Blue light’ to ‘opened again’. How does the writer use language to present the aquarium as calm but mysterious? Support your analysis with evidence from this paragraph. You may consider words and phrases, language features and techniques, and sentence forms.