You should use Dual blend anytime that Ionized Ammonia & Toxic Un-ionized Ammonia are present in the pond. Dual Blend also removes toxic nitrites and nitrates but the beauty is that it works in very cold water (as low as 35 degrees F). If you think your bio-filter needs a boost add Dual Blend™ which will improve the efficiency of your filter, regardless of its size, by aiding in reducing the heavy Bio-load during the summer months. Plus, it will give your filter the extra boost it needs to keep your pond water non-toxic during those hot summer spikes of ammonia and nitrite, by removing excess ammonia and aiding in reducing fish waste Reduces bottom sludge build-up all year. You will have much healthier fish when the water is clean. Maintains excellent water quality all year round and also reduces spring clean-up. Can be use all year.