behaviour
Group size and social requirements
Group size is a welfare requirement, not a preference. Shoaling species held in small numbers hide, lose colour, feed poorly and — in several species — redirect intra-group aggression onto other fish, which is why small groups of tiger barbs and serpae tetras cause so much more trouble than large ones.
Short answer
Many aquarium fish are obligately social, and a group below their threshold produces sustained stress behaviour rather than merely a quieter tank.
Shoaling and schooling are not the same
A shoal is a loose social group. A school is a coordinated, aligned formation. Rummy-nose tetras genuinely school; most 'schooling' tetras in trade shoal. Both need numbers, but a true schooling species needs the horizontal space to swim as a formation.
Practical minimums
- Six is a common floor and is often quoted as if it were sufficient for everything. For many species it is the point at which behaviour becomes acceptable, not good.
- Species with strong intra-group aggression — tiger barbs, serpae tetras — need ten or more, because the hierarchy needs somewhere to distribute itself.
- Bottom-dwelling social species — corydoras, kuhli loaches, zebra loaches — need groups too, and are routinely sold as single 'cleaners'.
- A bigger group needs a bigger tank. Increasing the group in a tank that is already too small is not a fix.