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Little is known regarding the ionoregulatory abilities of zebrafish exposed to soft water despite the popularity of this model organism for physiology and aquatic toxicology. We examined genomic and nongenomic changes to gills of zebrafish as they were progressively acclimated from moderately hard freshwater to typical soft water over 7 days and held in soft water for another 7 days. Gills were sampled daily and mRNA expression levels of gill Na⁺-K⁺-ATPase (NKA) α1a subunit, epithelium calcium channel (ECaC), carbonic anhydrase-1 and 2 (CA-1, CA-2), Na⁺/H⁺ exchanger (NHE-2), V-type proton (H⁺)-ATPase, and copper transport protein (CTR-1) were quantified by real-time PCR. Changes in enzyme activities of gill NKA were determined and protein levels of NKA and ECaC were quantified by Western blotting. Levels of mRNA for ECaC increased fourfold after day 6, with an associated increase in ECaC …