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In this study, we developed a toxicity model predicting the long‐term effects of copper on the reproduction of the cladoceran Daphnia magna that is based on previously reported toxicity tests in 35 exposure media with different water chemistries. First, it was demonstrated that the acute copper biotic ligand model (BLM) for D. magna could not serve as a reliable basis for predicting chronic copper toxicity. Consequently, BLM constants for chronic exposures were derived by multiple regression analysis of 21‐d median effective concentrations (EC50s; expressed as Cu²⁺ activity) versus physicochemistry from a large toxicity dataset and the results of an additional experiment in which the individual effect of sodium on copper toxicity was investigated. The effect of sodium on chronic toxicity (log K_NaBL = 2.91) seemed to be similar to its effect on acute toxicity (log K_NaBL = 3.19). However, in contrast to the acute BLM, no …