Five days. Japanese green powder is a kind of quick-acting fish medicine with strong efficacy, which can kill pathogens and pollutants quickly after entering the fish tank until it fails after 5 days. Therefore, Japanese green powder will be ineffective after 5 days in the fish tank. Japanese green powder is a common disinfectant for fish tanks. Its main components are chemicals such as chlorine and hydrogen peroxide, which can kill harmful substances such as bacteria, viruses and parasites in fish tanks.