Guppies are the most widely kept tropical fish globally. Competitive guppy breeding for show qualities has created welfare concerns about extreme phenotypes, while guppies are also extensively used in research.
Show guppy welfare is compromised by the same selective breeding logic as extreme dog breeds: phenotypes created for human aesthetic preference cause functional impairment and chronic welfare costs. Enormous tails impair swimming and are easily damaged in even well-maintained tanks. In research contexts, guppies are often housed in small, barren containers without social complexity appropriate to a highly social species. Their global introduction as an invasive species has caused enormous welfare harm to native aquatic communities across tropical regions.