I've been keeping freshwater ornamental fish for almost fifteen years, and in that time I've kept many schooling fish. Schooling fish, especially tetras, barbs and corydoras catfish, are the highlights of many fish tanks. There's just nothing quite like the sight of ten beautiful, identical fish moving together as one organism. But what happens when you keep schooling fish by themselves? Would it, for example, be okay to keep just one or two neon tetras, or a single tiger barb?
Many people assume that it's just fine to keep schooling fish alone, because they assume that fish can't get lonely. In my early days of fishkeeping, I made that mistake. I thought that a simple organism with such a tiny brain can't feel deep, "human" emotions like loneliness. And, in a way, they can't. I highly doubt that minnows are capable of feeling loneliness the way that we feel it, but keeping schooling fish alone can still seriously harm your fish.
Schooling fish kept alone, or in schools that are too small, have a tendency to become depressed. You may be rolling your eyes, but bear in mind that depression is a set of biological symptoms, including sluggishness and appetite loss. It doesn't mean your fish will experience an existential crisis and require a daily dose of Zoloft. Depression can be fatal for fish, because they may stop eating and swimming enough to stay healthy.
Stress is another problem that tends to affect schooling fish kept alone--and, again, this is a reference to a physiological condition, not a human emotion. Schooling fish kept alone have high levels of body stress hormones and may experience symptoms like stunted growth, infertility, heart problems, and susceptibility to disease. These symptoms can also cause a fish to die prematurely.
Schooling fish kept alone won't die instantly, so that has led many beginning aquarists to believe that it's perfectly safe and healthy for them. There will always be a few people with anecdotes about an unusually resilient fish who lived alone for five years with no problems. But, statistically speaking, a schooling fish kept by itself is much more likely to suffer from stress and depression, and may have a shortened lifespan as a result.
Another problem that tends to occur when schooling fish are kept alone is aggressive behavior toward tank mates. Tiger barbs are a perfect example of this. They have a reputation for being fin-nippers and attacking other fish, but they're actually quite peaceful when kept in suitably large groups (of eight fish or more). Schooling fish establish a pecking order and tend to more or less leave their tankmates alone when they're kept in groups, but lonely schooling fish will take out their natural intra-species aggression on slow-moving, long-finned tankmates.
If you don't have a large enough tank to keep an entire school of fish, stick to fish that do fine as solitary fish or in pairs or trios. Dwarf gourami, white cloud mountain minnows, Siamese fighting fish, and livebearers are all good beginner-friendly options that you might want to consider if you don't have enough room for more than three fish. Keeping a schooling fish alone simply isn't a good option.