The harlequin rasbora is a schooling fish; it should be kept in schools of eight to 10 individuals. Schools of even larger numbers make for a beautiful display. You can keep harlequins with any fish as long it’s not large and predatory. It will not nip at or quarrel with any other species. Some potentially good tankmates may include cardinal tetras, bettas, neon tetras, small barbs, dwarf gouramis, danios, other small rasboras, and cory catfish.