I am becoming a fish geek. No, that smacks of denial – I am a fish geek. I have been building a little world for tropical fish for the past few months, something to keep me off the streets. A compulsive person, I need something vaguely less soul-destroying than dial-a-pizza sex to be compulsive about – so a fish tank has become my latest obsession.

Every hobby has its own intricate rules and jargon – and I’ve learned all about this one, as I’ve learned most things recently, over the Internet, in the newsgroups.

I’ve learned all about the nitrogen cycle, the natural biological cycle that turns fish waste in the water into plantfood – and how long it takes for the helpful bacteria to grow. I’ve learned to debate the merits of injecting carbon dioxide into the water to reduce its pH, and whether or not pH is the most important factor in keeping Amazonian freshwater fish.

I’ve absorbed the fact that, like most things, there are experts with conflicting advice on practically every aspect of fish keeping, and that rules can be successfully broken, with fish thriving in environments that are not supposed to be suitable, and also that the most meticulous planning can still result in the little critters going belly-up, their passing a testament to the meaninglessness of things.

Create a biotope – the technical word for a living environment – and you learn a lot about the mechanics of life. What is curious is that personality is genetic, when it comes to fish.

Each species of fish has its own distinct characteristics – playfulness, curiosity, laziness, aggression, mating behaviour, tolerance and sensitivity. Watching fish school together, swimming as if part of a larger beast, and no matter how one can find technical explanations for the way the fish sense the position and direction of their fellows, it remains a glorious mystery how individual and collective experience are fused. I know I’m anthropomorphising wildly, but hey, someone’s got to do it.

My two cats, hitherto docile and sweet-natured beasts, have surprised me. To an extraordinarily tedious degree, every friend to which I’ve mentioned my fish-keeping plans over the past few months has joked about keeping a strong lid on the tank. Each time, I’ve smiled weakly and said that it wouldn’t be a problem. How wrong I’ve been.

The cats have undergone a complete personality change. Far from treating the tank like another television screen, it’s as if they have become possessed by the same wild hunting instinct that so overtook me sexually last year. They have set up residence beside the tank on the bookshelves, one on each level, with eyes fixed firmly on the flickering critters darting through the watery jungle.

Their intensity is unsettling in its familiarity – it’s the gaze of the cruiser, peering coldly into the woods waiting for an opportunity for a basic need to be met, that isn’t about hunger or desire, but something even more primitive.

To my knowledge, the cats have not managed to affect the fish at all. They paw earnestly at the glass whenever a minnow darts past, miaowing guttural moans. Buster, the male, has perfected a grip on the lid so his head hangs over the side of the tank, and he manages to hold his upside-down stance for up to ten minutes before either falling off or feigning disinterest by washing and settling down on the warm lid.

Portia, usually lazier than her brother, took a giant five-foot leap the other morning from a chair to reach the fish – but hadn’t counted on the fact that she can’t pass through glass, bashed her face right into the side of the tank, and came down to earth with a clatter. It took a while for her to regain her dignity amidst my guffaws.

However, a suspicion remains that their instincts have triumphed. The morning after the first fish arrived, of seven minnows, only three were left. No sign of bodies. I spent the entire day with a torch going over every inch of the 4 foot tank. I shone a light through the pipes to the filter pump, which I dismantled. I even used an old bicycle pump to squirt water through the carefully constructed Blue-Peter-esque tunnel under the gravel to flush out any corpses, in the unlikely event that all four had committed hari-kiri together underground. Nothing. No evidence that they ever existed.

It quite unsettled me. Having spent so much time getting everything right for them, creating the perfect home, their disappearance has ruffled me in a subtly undermining way. It gets me where I hurt – my capacity to nurture. Something basic in me ached when they disappeared, to a greater extent than I thought possible. It reminded me of when I discovered a golf-ball sized lump in Portia’s belly, and found myself distracted with worry. It turned out to be benign, but it was touch-and-go whether she would recover.

I have always believed that people’s sentimental attitudes towards animals (particularly strong among the English) is suspect, a projection of emotional dependency and vulnerability onto creatures that know nothing like the level of distress that a human child can undergo. They are shutting down care homes for children all over London now due to lack of funds, as the nation turns on in its millions to Rolf Harris and his ilk tending to sick animals. If an animal is being badly fed and neglected, the RSPCA has the legal right to enter a building on suspicion alone – if a child is in a similar situation, the state, for all the popular conceptions of invasive social workers, is relatively powerless to intervene unless the damage is visible.

Come to think of it, Portia was sick the morning the fish disappeared.

There’s nothing remotely sentimental about animals. Only we humans are that weak.