A study by researchers from the Universities of Tübingen and Athens has found that the toxicity of chemicals in lakes and rivers may vary by several orders of magnitude depending on the water's acidity. Professor Heinz Köhler from the University of Tübingen's Institute of Evolution and Ecology, working with colleagues from Athens and the German Environment Agency, tested the effect of 24 substances, most of which are used as drugs, on the development of fish embryos in realistic scenarios.