Critical swimming speed (Ucrit) predicts the maximum sustained swimming velocity that various fish species are able to sustain for prolonged periods. The objective of this study was to determine the Ucrit of Afrotropic ecozone fish, determine oxygen consumption at Ucrit and relate the resulting optimum flow requirements to effective movement through fishways under South African flow conditions. The selected fish species were Coptodon rendalli, Tilapia sparrmanii, Pseudocrenilabrus philander, Oreochromis mossambicus and Enteromius trimaculatus. Ucrit and oxygen consumption (MO2) were measured in a swim respirometer at 5-min intervals, at increasing velocity increments of 0.5 cm·s−1 until volitional exhaustion. No significant differences were seen in the Ucrit values between C. rendalli, T. sparrmanii and P. philander, but all species significantly differed from O. mossambicus and E. trimaculatus, which had the highest Ucrit (17.6 ± 1.5 bl·s−1 and 18.2 ± 2.8 bl·s−1). Size plays an important role in the swimming performance of fish, with larger fish able to sustain a greater velocity, which was specifically true for O. mossambicus in this study. Additionally, smaller fish consumed more oxygen during swimming and therefore used more energy, experiencing relative physiological strain. Based on these data, flow respirometry was shown to be a useful tool to determining prolonged swimming abilities of South African fish species, and can help inform the structure and flow rates of culverts and fishways.