at time zero, and every hour for the next 3 h, for subsequent measurement of Na+ concentrations. The same fish were left overnight
to acclimate. After 24 h, fresh aquarium water was added using
partial replacement to gently flush out experimental tanks while
minimising any stress to the fish. Once the overnight water had
been adequately flushed, a new volume of experimental water was
added to the exposure tanks and an identical period of sampling
was undertaken (i.e. water samples taken at time zero and every
hour for the next three hours, 24 h after initial handling)