___________ acts as a powerful oxidizing agent, and it is toxic to fish at concentrations of less than 0.05 mg/L

• A. chloride
• B. chlorine
• C. lime
• D. methane

Answer: (B)

Chlorine acts as a powerful oxidizing agent and is toxic to fish at very low concentrations. In water, chlorine exists mainly as hypochlorous acid (HOCl) and hypochlorite (OCl–), which are highly reactive and oxidize cellular components. This oxidative attack damages gill tissues and disrupts ion regulation, leading to respiratory distress and, at low concentrations like 0.05 mg/L, potential lethality for fish. Because of this toxicity, aquaculture environments require careful management of residual chlorine, often using dechlorination before water reuse or discharge.

Chloride, the Cl– ion, is not an oxidant and is relatively harmless at typical water levels. Lime is calcium hydroxide, a base used to adjust pH, not an oxidant. Methane is a hydrocarbon, not involved in disinfection or oxidation in this context.