No matter how small your goldfish was when you first brought him home, chances are good he won't stay that way for long. Goldfish have been known to grow upwards of 12 inches, so as your goldfish grows, you may need to upgrade to a bigger tank.
Net your goldfish carefully and transfer him to a temporary tank while you prepare the new tank. The temporary tank should be kept at the same water temperature as your original tank.
Clean your new goldfish tank by rinsing it well with fresh water and wiping it out with clean paper towels. Your new tank should be at least 50 gallons in capacity to support several goldfish -- goldfish produce a great deal of waste so they require large tanks in order to thrive.
Line the bottom of your goldfish tank with your preferred substrate to a depth of one to two inches. If you use some of the substrate from your original goldfish tank you will also be transferring some beneficial bacteria to your new tank. This will help your new tank cycle more quickly.
Fill your tank about three quarters of the way with fresh tap water. Try to match the water temperature of your existing tank so your aquarium heater will be able to stabilize the temperature more quickly.
Position your aquarium decorations in your new goldfish tank. You may choose to use your existing decoration or use a combination of old and new, just be sure to rinse any new decorations well to remove dirt and dust.
Fill your tank the rest of the way with tap water and dose it with an aquarium water conditioner to remove chlorine and other chemicals that the water may have been treated with. Follow the instructions on the bottle to determine the proper dosage for your tank capacity.
Install your aquarium heater and filter in your new tank. If your new tank is significantly larger than your old goldfish tank you may need to upgrade your equipment as well -- read the product information that came with your heater and filter to determine what size tank they are rated for.
Let your aquarium sit with your equipment running for two weeks or until the tank has fully cycled. If you introduce goldfish into the tank before the nitrogen cycle has been established, they may be poisoned by high levels of ammonia.
Test your aquarium water once a day as the tank cycles. When your test results show an ammonia and nitrite level of zero and nitrates begin to register on the test, it is safe to assume the tank has cycled.
Introduce your goldfish into the upgraded tank by putting them in a plastic bag full of water from the temporary tank. Add a small amount of water from the new tank and float the bag in the new tank for 30 minutes before netting the fish and releasing them into the tank.
- If you skip the final step in slowly introducing your goldfish to the new tank your fish could be shocked by a drastic change in water temperature or water chemistry. Floating your fish in a bag in the new tank gives them time to acclimate slowly to the new conditions.