Ecological Systems and Devices Annotation << Back Influence of Water Composition on the Toxicity of Potassium Dichromate for Danio Rerio Eggs and Larvae in Acute Tests Vorobieva O.V., Ryzhkova I.V., Erikov N.A., Samoilova T.A., Oganesova E.V. The effect of the reference comparison toxicant, potassium dichromate, on the eggs and larvae of Danio rerio was studied in different types of water. The fi sh were grown in tap water with a hardness of 2.5 °dH and in an artificial water with a standardized composition and simulated hardness parameters (0.06 °dH, 2.5 °dH, and 4.16 °dH). The type of water and water hardness did not influence the survival of eggs and larvae of D. rerio in control groups without the toxicant, but they significantly affected the toxicometric parameters of the fi sh. The maximum non- effective concentrations for potassium dichromate were 400 mg/L in tap water with a hardness of 2.5 °dH, 100 mg/L in artificial water with a hardness of 2.5 °dH, 200 mg/L in artificial water with a hardness of 0.06 °dH and 4.16 °dH. The LC50 values in tap water were statistically significantly higher than in artificial water. Despite a 70-fold difference in hardness values, the LC50 values in artificial environments did not differ significantly. The different water types showed significant differences in total mineralization and the concentrations of Na+ , K+ , Cl- , and HCO3 – . Although the concentrations of these ions are not regulated by bioassay protocols, they can have a substantial impact on the results of biotesting. Therefore, the water composition used in bioassays with D. rerio caviar can markedly influence the derived LC50 and maximum non-effective concentrations. Keywords: toxicity, bioassay, water hardness, Danio rerio. DOI: 10.25791/esip.9.2025.1545 Pp. 45-53.