Goldfish are relatively tough compared to other popular tank fish, but this doesn't mean you can just toss your new pet into a new environment and expect him to thrive. To give your goldfish the best start in his new home, take the time to let him acclimate by floating the bag in the tank before releasing him.
The reason for floating the plastic bag in the tank when bringing home your new goldfish is to let him slowly acclimate to the new tank temperature. Dumping him into water that is cooler or warmer than his current surroundings could cause him to go into shock and is hard on his little body. Letting the bag sit in the tank lets the water inside gradually match the temperature of the aquarium. This slow change is much easier on your new goldfish. Ideally, the temperature in the tank should be between 62 and 68 degrees Fahrenheit.
The amount of time it takes for the water in the plastic bag to become the same temperature as the tank depends on the amount of water in the bag and the difference in temperature. In most cases, 15 minutes is sufficient for the temperatures to become equal. If there is a lot of water in the bag, however, or if you can feel a large temperature difference, it may require more time. It's not an exact science, but if you can feel a difference between the water in the bag and the water in the aquarium, give it more time to adjust.
After the water temperature has equalized, open the bag and transfer 1/2 cup aquarium water into the bag and let it sit for minute or two, then add another 1/2 cup water from the tank and wait again. Repeat this process several times until the bag is filled with aquarium water. Once your goldfish is surrounded mainly by water from the tank, gently lift him out of the bag with a net and release him into the tank. Alternately, you can open the bag and release him, but you may also release any dirt or bacteria contained in the water from the store.
Rapid changes in light can stress your new goldfish, so don't carry him around in just the clear plastic bag when transferring him from the store to the car and the car to the house. Place the plastic bag in a paper bag or a small ice chest. Prepare the water in the tank about two weeks before bringing your fish home. This will give any chlorine plenty of time to dissipate, beneficial bacteria time to grow and sufficient time for the pH to become balanced. It will also give you a chance to ensure that all of your equipment, such as pumps and filters, are working properly before your fish's health depends on it.