Which statement about copper sulfate toxicity to fish in soft waters is true?

• A. It is more toxic in hard waters.
• B. Some species may be killed with applications greater than 0.25 ppm
• C. Trout and carp are resistant
• D. It only affects aquatic plants.

Answer: (B)

In soft waters, copper becomes more bioavailable to fish because there are fewer competing ions like calcium and magnesium to block uptake. That means copper sulfate can reach toxic levels more easily, and some fish species can be killed at concentrations around or above 0.25 mg/L (ppm). So saying that some species may be killed with applications greater than 0.25 ppm captures this species- and water-hardness-dependent toxicity.

The other statements aren’t accurate for this context: copper sulfate is not more toxic in hard waters; it does not only affect aquatic plants—fish and other aquatic life are also harmed; and trout and carp are not universally resistant—sensitivity to copper varies by species and water conditions, with soft waters generally posing higher risk.