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Figure out how much water your bowl holds. Figure out how much new water you'll need. Fill the clean container with fresh water. Treat the water using a water treatment kit. Cover the container and let the water age overnight.
In order to determine how much clean water you'll need to prepare, you first need to gently pour the bowl's current water and the fish into a clean, soap-free bucket or container. Then, mark the water level on the outside of the bucket. When you clean your fish bowl, you'll need to replace 40-50% of the total volume of water that it holds. After you've marked the initial water level, use a clean cup to start putting the water back into your bowl. Once you've removed about half of the water, mark the new water level on the outside of the bucket and transfer the fish back into the bowl with a net. Once your fish is safely back in its bowl, rinse out the container you were just using. Don't use soap to clean out the container, because this will be harmful to your fish. Fill the container back up to the halfway mark you made with fresh water. At this point, read and follow the directions on the label of your water treatment kit. Add the appropriate number of drops to the water that's in the container. You can find water treatment for fish bowl water at any major pet store. The treatment will eliminate chlorine, chloramines and other substances that are toxic to fish. This is a necessary step in ensuring that the water will have the proper balance of chemicals to keep your fish healthy. Cover the container with a lid to prevent airborne contaminants from making their way into the water. Let the container sit overnight so that the water can "age" and reach room temperature.