The domestic, agricultural, industrial, technological and medical applications of potentially toxic elements (PTEs) have led to global pollution in all environments. In this study, the cnidarian Hydra attenuata was exposed individually and to a mixture of 5 metals (copper, iron, manganese, zinc and nickel) at environmentally relevant concentrations (1×) within the Clyde estuary, Scotland and incremental concentrations ranging from 0.0001× to 1000×. Toxicity was investigated using morphology, attachment, hydranth number and feeding behaviour as endpoints. When exposed individually, Cu, Mn and Fe significantly reduced Hydra morphology, feeding and attachment at environmentally relevant concentrations. Hydra mortality was measured, having an LC50 of 0.045× (for the environmentally relevant mixture of metals) and Cu 0.5 mg/l, Fe 3 mg/l, Mn 2 mg/l, Zn 0.1 mg/l, Ni 0.5 mg/l for each element exposed individually. The PTE mixture incurred a significant decrease (p ≤ 0.05) in morphology at 0.0001×, with 100% mortality at 0.1× (containing a concentration of Cu 0.05 mg/l, Fe 0.3 mg/l, Mn 0.2 mg/l, Zn 0.01 mg/l, Ni 0.05 mg/l) and a toxicity threshold (TT) of 0.000005×. Both copper and iron when exposed individually to the concentration of their respective metals found in the environment resulted in 100% mortality for all Hydra exposed. These results indicate that the PTE mixture (including the individual concentrations of copper, iron, manganese and nickel) could potentially prove significantly toxic to the aquatic environment.