Routine metabolic rate (RMR, mgO2 g−1 h−1) and critical oxygen concentration (Pc, a hypoxia tolerance indicator, mgO2 L−1) were determined in larvae and juveniles of round crucian carp, Carassius auratus grandoculis Temminck & Schlegel 1846, by measuring oxygen consumption at 15°C, 20°C, and 30°C. In addition, the dependence of RMR and Pc on fish body weight (W, g) and temperature (T, °C) was examined to construct minimal mathematical models. RMR depended on W and showed smaller values in larger individuals. RMR was different among the three temperature conditions and showed higher values at higher temperatures. Pc was significantly related to W and was low in larger individuals; that is, larger individuals had a higher hypoxia tolerance. In contrast, Pc was independent of temperature, implying that seasonal temperature fluctuations do not cause seasonal disequilibrium of hypoxia tolerance in round crucian carp. The RMR and Pc models were RMR = 0.0674W−0.193e0.0562T and Pc = 1.35W−0.107, respectively. The metabolic information clarified in this study is essential for habitat quality assessments and aquaculture management of this species.