In the changing rooms, I overhear a conversation between two men.

'What more could you ask for?' says one.

The other laughs. Then, after a pause, he says, 'A body that can deal with change.'

'Is there such a thing?'

I leave the room to take a shower. I remind myself that water is a deep blessing, whenever and wherever you find it. I have never been healthier, but I have been feeling irritable for several days, no obvious reason.

When I return to my locker the two men are just about to leave. They are still talking. The conversation must have moved on by now. One of them says, 'Don't ask for too much, or they'll take it away.' I don't who or what he's referring to.

In the lobby, I wait for my girlfriend and watch tropical fish billow and drift in a tank of cosmic hues. I cannot tell if the coral is organic or made of fluorescent plastic. Some sections look immobile at first glance, but when you watch more closely you can see small filaments quivering in the thermal currents. The effect is faintly hallucinogenic. A crab with huge whiskers sidles from one side of the enclosure to the other. The glass is always exceptionally clean. I wonder who maintains it, and how. I wonder if the fish and the crab and whatever else lives in there get pumped out every time their home is scrubbed, or if the cleaning takes place whilst they're still there. They might not even know.

There must be periods of time when a fish has died but nobody has noticed; when it floats along the surface, white-eyed, colder than it was in life, nudged by bubbles. Maybe customers have had to inform the staff on such occasions. 'Excuse me, but I think one of your fish is dead.' Or perhaps part of the staff's early morning routine is to check the tank, make sure everyone is still swimming. Gyms are a place for the living. The dead are never healthy.

I remember a bathroom in which there are small soap dishes filled with both soap and shells. Some of the soaps are designed to look like tropical fruit - pineapples, bananas, watermelons - but each is wrapped in thin plastic. They could be boiled sweets. I can't imagine them ever being opened.

All fish kept in a tank look trapped. It's impossible not to think of it as a prison - cramped, beautiful, a distraction of finite, luminous forms. One silver-bellied, blue-finned resident looks like she's drifting without purpose, but I notice there are tiny particles floating through the water - I almost wrote 'the air' - and this fish gulps them down. They disappear. I don't know if she looks for them, but when they're there, she doesn't let them go.