In all five mixing tanks, 14 mg L−1 tri-sodium citrate (C6H5Na3O7*2H2O; pro analysis from VWR International) was added in excess to complex and eliminate the toxic effects of aluminium (Fivelstad et al., 2004). Carbon dioxide of 99.9% purity was added by diffusers to four mixing tanks (not to the mixing tank for the control group). Sodium bicarbonate was also added to the water in these four mixing tanks, to stabilise pH, from a stock solution by use of a peristaltic pump (Alitea C6XV). The carbon dioxide concentration during the first 7 days (days 0–7) of the experiment was consistently kept in the range 7–10 mg L−1 for the experimental groups to give the fish the opportunity to acclimate to high carbon dioxide levels. The low carbon dioxide group was exposed to a slightly higher range (7–15 mg L−1) for the rest of the experiment. On day 7, the carbon dioxide partial pressure was increased in the medium, high and very high carbon dioxide groups to 6.5 mmHg (17 mg L−1), 9.5 mmHg (25 mg L−1) and 10.9 mmHg (29 mg L−1), respectively. Both the medium and high groups were run at the final levels for the rest of the experiment (see Table 1). In the very high group, the concentration was further increased on day 23 (to 14.2 mm Hg, i.e. 38 mg L−1). The target levels for carbon dioxide were generally reached within about 12 h (on day 0, day 7 and day 23). The carbon dioxide concentration of the control group was about 2 mg L−1 from day −14 until the end of the experiment.