How to keep your Siamese fighting fish happy. Credit:Rebecca Hallas When I sought answers from scientists, I assumed that they would find the question preposterous. But they did not. Not at all. It turns out that not only can our gilled friends become depressed, but some scientists consider fish to be a promising animal model for developing anti-depressants. New research, I would learn, has been radically shifting the way that scientists think about fish cognition, building a case that pet and owner are not nearly as different as many assume. The neurochemistry is so similar that it's scary," said Julian Pittman, a professor at the Department of Biological and Environmental Sciences at Troy University in Alabama, where he is working to develop new medications to treat depression, with the help of tiny zebrafish. We tend to think of them as simple organisms, "but there is a lot we don't give fish credit for", he said.

Dr Pittman likes working with fish, in part, because they are so obvious about their depression. He can reliably test the effectiveness of antidepressants with something called the "novel tank test". A zebrafish gets dropped in a new tank. If after five minutes it is hanging out in the lower half, it's depressed. If it's swimming up top - its usual inclination when exploring a new environment - then it's not. In Dr Pittman's lab, researchers induce depression in a fish by keeping it drunk on ethanol for two weeks, then cutting off the supply, forcing it into withdrawal. Give a depressed bottom lurker an effective antidepressant and within two weeks it will swim up to the top again. The severity of the depression, he said, can be measured by quantity of time at the top versus the bottom all of which seemed to confirm my suspicions about Bruce Lee.

All of this, of course, may sound fishy to any of the one in six people who has experienced clinical depression. How could a striped minnow relate to what you've been through? Is "depression" the right word? While scientists have used animals, such as mice, to study emotional problems for decades, the relevance of those models to human experience is sketchy at best. There's the obvious issue that "we cannot ask animals how they feel", said Diego Pizzagalli, the director of the Centre For Depression, Anxiety and Stress Research at Harvard Medical School. Though researchers may find parallels in serotonin and dopamine fluctuations, neither fish nor rat can "capture the entire spectrum of depression as we know it", Dr Pizzagalli said. There is a heated debate in the fish research community about whether "anxious" or "depressed" is a more appropriate term.

But what has convinced Dr Pittman, and others, over the past 10 years is watching the way the zebrafish lose interest in just about everything: food, toys, exploration - just like clinically depressed people. "You can tell," said Associate Professor Culum Brown, a behavioral biologist at Macquarie University in Sydney, who has published more than 100 papers on fish cognition. "Depressed people are withdrawn. The same is true of fish." The trigger for most domestic fish depression is probably a lack of stimulation, said Victoria Braithwaite, a professor of fisheries and biology at Penn State University, who studies fish intelligence and fish preferences. Study after study shows how fish are defying aquatic stereotypes: some fish use tools, others can recognise individual faces.

"One of the things we're finding that fish are naturally curious and seek novel things out," said Dr Braithwaite. In other words, your goldfish is probably bored. To help ward off depression, she urges introducing new objects to the tank or switching up the location of items. Dr Brown agrees, pointing to an experiment he conducted, that showed that, if you leave a fish in an enriched, physically complex environment - meaning lot of plants to nibble on and cages to swim through - it decreases stress and increases brain growth. The problem with small tanks is not just the lack of space for exploration, said Dr Brown, but the water quality tends to be unstable and there may not be sufficient oxygen. "A goldfish bowl for example is the worst possible situation," he said.

If you own fish, you might want to consider where Dr Brown keeps his: an extensively landscaped 1.8-metre tank. He recommends a "two-foot [60-centimetre] tank with lots of plants and stuff" for your average Siamese fighting fish. The last time a guest posted Bruce Lee to Instagram, he was looking good and lively. Perhaps that new green leaf in his bowl had provided the enrichment he craved. But then, my heart sank. The internet produced photos of other Bruce Lees from the same hotel in several colours - red, blue and purplish. I wondered whether the monotony would eventually drive these Bruces to hover, immobile, near their transparent rocks. The New York Times