This study evaluated the efect of fsh total length (LT) and three water temperatures (10, 15 and 20 °C) on the critical swimming speed (Ucrit) of the species Percilia irwini (2.9–6.3 cm LT), Cheirodon galusdae (3.4–5.5 cm LT), and Trichomycterus areolatus (4.0–6.3 cm LT). An Ucrit estimation model was constructed for each species as a function of temperature and size. The results showed mean Ucrit for P. irwini of 44.56, 53.83 and 63.2 cm ­s−1 at 10, 15 and 20 °C, respectively: 55.34, 61.74 and 70.05 cm ­s−1 for C. galusdae and 56.18, 63.01 and 71.09 cm ­s−1 for T. areolatus. Critical velocity depended on the interaction between species, body length and water. The swimming performance increased signifcantly with rising temperature in all three species. The velocity also increased with greater fsh total length. After controlling for fsh total length, velocity also increased with higher temperature in the three species. This research is relevant to small fsh species that require conservation measures.