Concept:Fish pond diseases are often triggered by avoidable human errors in pond care.Explanation:Poor management practices include pollution of the pond water and the use of infected or contaminated equipment.These practices introduce harmful organisms and toxins that can make fish sick.Liming and harvesting are normal pond operations when done correctly.Mixed stocking can stress fish, but it is not the main direct cause named in this question.Answer:The correct answer is C. poor management practices.