Water hardness affects metal uptake and toxicity in fish. Calcium is the most important ion determining water hardness. Fish are one of the most important food supplies for humans. Thus, Oreochromis niloticus were exposed to Cr6+, Ni2+, Zn2+ and Hg2+ in differing Ca2+ levels (30, 60 and 120 mg Ca2+/L) and subsequently serum biochemical parameters and muscle food quality were investigated. In acute experiments (3 days), fish were exposed to 30 mu M of Cr6+, Ni2+ and Zn2+, and 0.3 mu M of Hg2+, though they were exposed to 10 mu M for Cr6+, Ni2+ and Zn+2 and 0.03 mu M for Hg2+ in chronic experiments (30 days). Data showed that there were significant (p < 0.05) alterations in the levels of glucose, alanine transaminase, aspartate transaminase, cholesterol and tri-glyceride in the serum, most alterations being at the lowest calcium level. Serum ion levels (Na+, K+, Ca2+) also altered significantly, but only in chronic exposures. There were significant variations in the quality of muscle in both durations, as total protein, lipid and raw ash levels altered significantly (p < 0.05), though no significant change occurred in the water content of muscle tissue. This work demonstrated that metals affected the serum biochemistry and muscle food quality of Oreochromis niloticus, emphasizing the stress that fish could face in metal-contaminated waters.
