The biotic ligand model (BLM) helps predict how water chemistry affects metal toxicity in aquatic animals. It focuses on how metals bind to specific receptors in organisms, causing harm. By considering factors like metal speciation and competition with protective cations, the BLM can explain why certain ligands and water hardness influence metal toxicity. A copper version of the BLM was developed and tested on freshwater fish and Daphnia. The model accurately calculated lethal concentrations for these organisms, showing how copper toxicity can be predicted and understood in different aquatic environments.