The purpose of this investigation was to measure the in situ health of selected biota with respect to a large-scale application of an aluminiumamended zeolite (Z2G1) in Lake Okaro, New Zealand. We hypothesized that, based on previous laboratory toxicity testing, the Z2G1 application would have minimal or no significant health effects on lake fauna. To test this hypothesis, two sampling events were timed around the Z2G1 application to examine the health of two finfish species (Oncorhynchus mykiss and Gobiomorphus cotidianus) and crayfish (Paranephrops planifrons). Additionally, the same species were sampled from the neighbouring Lake Rerewhakaaitu in order to delineate seasonal and lake-specific effects. A general lack of aluminium accumulation in animal tissues following the application indicated that Z2G1-derived aluminium was not readily bioavailable. An unexplained osmoregulatory disturbance, indicated by increased blood plasma ion concentrations in Lake Okaro trout, was observed following the application, but could not be directly attributed to Z2G1 exposure. Changes in fish haematology over time and between lake populations were interpreted as physiological responses to seasonal changes in lake conditions. Using a broad range of specific and non-specific endpoints it was concluded that there were no obvious negative impacts on fish health resulting from Z2G1 exposure.