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Quarantining your new fish will ensure they are healthy and will not introduce disease or illness into your existing tank. The quarantine tank should be at least 5 to 10 gallons, with a sponge filter that has been in a tank with fish. This will ensure the filter contains good bacteria to populate the tank. The tank should also have a heater, an aquarium light, and a cover. If you are an avid aquarium owner, you may already have a quarantine tank set up. You should keep the quarantine tank clean and prepare it before you buy new fish for your regular tank. Once you get your quarantine tank set up, you can introduce your new fish to the tank through acclimatization.  Start by placing the unopened plastic bag in the tank for 15-20 minutes. This will give the fish time to start getting used to the water in the quarantine tank. After 15-20 minutes, open the bag and use a clean cup to scoop an equal amount of water from the tank into the plastic bag. There should be 50% tank water and 50% pet store water in the bag. Avoid mixing water from the bag with the tank water as this could contaminate the tank water. Allow the bag to float in the tank for another 15-20 minutes. You can close the edge of the bag to keep in from spilling out. After 15-20 minutes, use a net to gently scoop out the fish and place them in the quarantine tank. You should observe the fish every day in the quarantine tank to ensure they are not carrying any disease or parasites. After two to three weeks in the quarantine tank with no issues, the fish is ready to be introduced to the main tank. Doing a water change will allow your new fish to get used to the nitrates in the water and avoid stressing out your new fish. This is an especially important step if you do not do regular water changes of your main tank water. To do a 25 to 30 percent water change, remove 25 to 30 percent of the tank water and replace it with dechlorinated water. Then, cycle through the water several times with your filter to ensure the nitrate balance in the tank water is correct. If you already have fish in your tank and are introducing a new fish to the tank, you should make sure you feed the fish first. This will make the fish in the tank less aggressive towards the new addition. Move around any rocks, plants, and hiding places in the tank to new spots. Rearranging the accessories before you introduce the new fish will distract the existing fish and remove any established territories that have been marked in the tank. This will ensure your new fish enter the tank on equal grounds and are not isolated from the other fish. Once your new fish have been properly quarantined, you should repeat the same acclimatization process you did with the quarantine tank for the main tank water. This will help the fish get used to the main tank water and ease into their new environment. Place the fish in a bowl or bag filled with the quarantine tank water. Let the bag or bowl sit on the surface of the main tank water for 15-20 minutes. Then, use a clean cup to scoop some of the main tank water into the bag or bowl. There should be 50% main tank water and 50% quarantine tank water in the bag. Let your fish sit in the bag or bowl of water for another 15-20 minutes. Then, use a net to gently scoop the fish out of the bag or bowl and place him in the main tank. Monitor the new fish over the next few weeks to ensure he is getting along with his tank mates and is not displaying any signs of illness or disease.

Summary:
Set up a quarantine tank. Place the new fish in the quarantine tank for two to three weeks. Do a 25 to 30 percent water change. Feed any fish in the main tank. Rearrange the accessories in the tank. Acclimatize the new fish to the main tank water. Introduce the new fish into the main tank.