Zebrafish are the most widely used fish in research. Despite their regulatory exemption from many animal welfare requirements, they deserve meaningful welfare consideration.
Zebrafish welfare occupies an uncomfortable gap: they are vertebrates with well-documented pain responses and behavioral complexity, yet regulatory exemptions covering larvae and the sheer scale of zebrafish research mean they receive minimal welfare attention. As obligate social shoaling fish, zebrafish in isolated tanks experience social deprivation that alters their stress physiology and behavior. This creates a scientific problem as well as a welfare problem: stressed, isolated fish show different biological responses than healthy, socially housed animals, introducing variability that compromises research validity. The welfare case for zebrafish improvement aligns with the scientific case: appropriate social housing, adequate water quality, and reduced unnecessary handling improve both welfare and research reproducibility. The 3Rs framework applies fully to zebrafish, and refinement of their care is both feasible and warranted.