Very few species of tropical fish are naturally solitary or pair forming and so most need to be kept as shoals or groups for best results. Tetras, barbs, rainbowfishes and catfishes such as Corydoras are found in vast numbers in the wild and will only exhibit natural behaviour when able to interact with a number of their own kind. Signs of unusual behaviour may include aggression towards other tankmates and an otherwise peaceful community species can exhibit unusual behaviour such as fin-nipping.
Many fish that exhibit marked differences in colouration are sold as pairs to ensure the sale of the somewhat more plain females but without females to impress, males often fail to display their bright colours. Species such as gouramis fall into this category and are best kept as multiple pairs in large well-planted aquaria. Amongst the few species that do need to be kept as pairs are some of the monogamous cichlids but even these prefer to select their own mates and are best kept initially in groups when young.
By contrast, most marine fishes are territorial and aggressive and should be kept singly in the case of average-sized aquaria. Some species can be kept in pairs or small groups but these are the exception and should ideally be seen together before purchase and introduced simultaneously for best results.