| Summary: | Fathead minnows, Pimephales promelas, and yellow perch, Perca flavescens, were transferred from moderately soft Lake Superior water (hardness similar to 45 mg/l as CaCO3) to very soft diluted Lake Superior water (hardness similar to 4.5 mg/l). Sulfuric acid was added in some treatments by means of a multichannel diluter. In very soft water, chloride cells proliferated in the gills, especially in the epithelium of the secondary lamellae. When exposed to acid, chloride cells were damaged and less abundant in the secondary lamellae, and blood osmolality was reduced at pH 5.0 for the minnows and pH 4.1 for the perch.
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