Ceriodaphnia dubia were cultured in four reconstituted water s with hardness and alkalinity concentrations ranging from soft to the moderately hard water that is required by whole‐effluent toxicity (WET) testing methods for culturing test organisms. The effects of these culture formulations alone and in combination with two levels of Cl⁻,SO²⁻₄, and HCO⁻₃ on reproduction of C. dubia were evaluated with the standard three‐brood test. Reproduction was significantly reduced when test waters had lower hardness than culture waters. However, reproduction was not significantly different when animals cultured in low‐hardness waters were exposed to moderately hard waters. The hardness of the culture water did not significantly affect the sensitivity of C. dubia to the three anions. Conversely, increased hardness in test waters significantly reduced the toxicities of Cl⁻ and SO²⁻₄, with HCO⁻₃ toxicity following the same pattern. Alkalinity exhibited no consistent effect on Cl⁻ and SO²⁻₄ toxicity. The physiological stress of placing animals cultured in moderately hard water into softer test waters might contribute to marginal failures of otherwise nontoxic effluents. The standard WET protocol should be revised to allow the culture of C. dubia under lower hardness conditions to better represent local surface water chemistries.