Water Hardness Can Reduce the Accumulation and Oxidative Stress of Zinc in Goldfish, Carassius auratus

We investigated the changes in toxicity stress in goldfish, Carassius auratus, under exposure to different concentrations of Zn and water hardness for 14 days. We analyzed the changes in water hardness and Zn accumulation after exposure. To investigate the stress levels, the expression of metallothionein, caspase-3 activity, NO activity, and total antioxidant capacity were detected. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assays were also performed to measure apoptosis in the liver. The results showed that compared to the control group, a more significant difference in the accumulation of Zn in body stress markers (metallothionein, caspase-3 activity, NO activity, and total antioxidant capacity) were observed with increasing Zn concentration and exposure time. Notably, at the same Zn concentration and exposure time, lower stress levels were discovered in the samples under harder water conditions. Finally, the TUNEL assay showed that Zn accumulation caused apoptosis and high water hardness could reduce the apoptosis. In conclusion, we found that high water hardness can influence the absorption of Zn, and alleviating the hardness levels can reduce the toxicity stress caused by Zn.