No! Never use chlorine-treated water with your fish tank! Chlorine is dangerous for your fish. But you can treat the water so that it's safe. One popular method of using chlorinated water for fish tanks is sodium thiosulfate. Sodium thiosulfate helps to remove chlorine from water, making it safe for an aquarium with water and fish. No matter the source of dechlorination, we always recommend using an aquarium water testing kit to ensure the water is safe.