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The individual effect of different major cations(Ca²⁺, Mg²⁺, Na⁺, K⁺, and H⁺) on the acute toxicity of zinc to the waterflea Daphnia magna was investigated. The 48‐h median effective concentration (EC50) in the baseline test medium (i.e., a standard medium with very low ion concentrations) was about 6 μM (Zn²⁺). An increase of Ca²⁺ (from 0.25 mM to 3 mM), Mg²⁺ (from 0.25 mM to 2 mM), and Na⁺ activity (from 0.077 mM to 13 mM) reduced zinc toxicity by a factor of 6.3, 2.1, and 3.1, respectively. No further toxicity reduction was observed when Ca²⁺ and Mg²⁺ activities exceeded 3.0 and 2.0 mM, respectively. Both K⁺ and H⁺ did not significantly alter zinc toxicity (expressed as Zn²⁺ activity). From these data, conditional stability constants for Ca²⁺ (log K = 3.24), Mg²⁺ (log K = 2.97), Na²⁺ (log K = 2.16), and Zn²⁺ (log K = 5.31) were derived and incorporated into a biotic ligand model (BLM) predicting acute zinc …