To dechlorinate fish water naturally, let the water sit for 24-48 hours before adding it to the tank. Use a dechlorinator treatment to neutralize chlorine and chloramines instantly. Another natural option is to use activated carbon or charcoal filters in the aquarium to remove chlorine and other chemicals. Boiling tap water for at least 15 minutes and then letting it cool also helps to remove chlorine. Rainwater can be used as a natural source of water after checking its quality. Adding live plants to the aquarium can also help to absorb chlorine and provide a natural environment for the fish. Regular water changes and using a water conditioner when adding new water will also maintain a healthy aquatic environment for your fish.