Shoaling fish feel safe in numbers and schooling fish swim together in coordinated groups; both need to be kept in a proper group (commonly six or more) or they become stressed and sickly.
Tetras, rasboras, danios, corydoras and many others are shoalers. Kept in ones or twos they hide, lose colour, and may turn nippy — behaviour often mistaken for the fish being "shy" or "aggressive."
Buying a real group is not optional decoration; it is a welfare need, and it is when these fish finally look their best.