Many aquarium fish naturally live in groups. One group of fish may range from as little as two or three to more than twenty.

Courtesy of Yoko Dam

In fish, group size can affect aggression, darting, shoaling behaviour, and neophobia. Small fish such as neon tetras and white cloud mountain minnows prefer larger groups and display shoaling behaviour in these larger groups. This decreases aggression and darting behaviour, which are linked to fear and stress. Larger fish like angelfish and tiger barbs also showed increased shoaling behaviour when placed in large groups. Keeping neon tetras, tiger barbs, and white cloud mountain minnows in large groups improves their welfare, but there seems to be no definitive proof of improved welfare for angelfish.

For a prospective owner of neon tetras, minnows, or tiger barbs, it would be better to keep the fish in large groups. It should be noted that the effects of grouping are species-specific. Only species that evolved to thrive in groups should be kept in groups. It would be detrimental and poor welfare to keep more than one male Siamese fighting fish in a tank, and thus hobbyists do not keep multiple male fighting fish in one tank. It is very important to learn about the species before buying it and keeping it in a home aquarium.