How to Upgrade an Aquarium: Moving Fish to a New Tank

Rasbora Fish in an Aquarium

Ezequiel Ferreira/EyeEm/Getty Images

If your floor can support the weight of added water, upgrading to a larger tank is always a great idea. Bigger aquariums are easier to maintain, and you can even keep more fish! Here are the steps to successfully transfer everything from your smaller, established aquarium to a new larger one. 

Preparing the Old Aquarium for Exit

If the aquarium has recently gone through a major event such as fish death, a replacement of the filter media, or new fish being added, it's important to wait a few weeks before undertaking this aquarium switch. This will allow the old aquarium to stabilize.

Beneficial bacteria colonize not only the filter media but also the gravel bed and all other hard surfaces of the aquarium, such as rocks and decorations. All fish, as well as all decor, equipment, and gravel, can be transferred without washing, but it's advisable to leave most of the old water behind.

Setting Up the New Aquarium

To support the new system, you'll need a bigger filter and heater. New equipment must be rated for the larger tank volume as the old ones won't be powerful enough for the new size. If you plan to use the old filter in the new tank, leave it running on the smaller tank to keep its beneficial bacteria alive.

Set up the new tank by filling it only three-quarters full of water. That will leave enough displacement volume to handle the new equipment, additional gravel, and any decorations. Also, run an air stone in the new tank to help dissipate any dissolved gases that may be present in your water supply. Let the new system run for 24 hours.

Testing the Water to Match

Both tanks will need to be heated at the same time to ensure equal temperature conditions so as to avoid shocking the fish. Later, your old heater can serve as your backup as aquarium heaters sometimes fail. 

Test and record the pH and water temperature of the old aquarium. After 24 hours of settling, test the pH and temperature in the new aquarium; compare your findings. Do not proceed with the transfer if there is a difference of more than two-tenths of a point of pH level or more than a two-degree difference in water temperature between the two aquariums.

Of these two parameters, pH is the most critical. Adjust the pH of the new aquarium up or down just as you would in the old one.

Moving the Gravel and Filtration

Use a clean cup to transfer the gravel from the old tank to the new one. Next, move all the rocks and decorations. If you plan to use the old filter in the new tank, move it at this time.

The filter from the old tank has beneficial bacterial colonies in the filter media. By running both the old and the new filter for a few weeks, you'll introduce plenty of beneficial bacteria into the new system. This extra filtration will also be helpful as the new aquarium becomes established.

Make sure all filtration is running before moving the fish. Treat the new aquarium as you would any new tank, which means that you should test the water for both ammonia and nitrite daily and perform partial water changes until they are at zero.

Moving Your Fish 

If the pH and the water temperature match exactly, you can transfer fish without needing to bag them; use a clear plastic pitcher with a barrier on top. Otherwise, you can bag the fish and acclimate them the same way you would with any fish you purchase and bring home.

To catch each fish, hold the pitcher underwater and use a small net to gently guide the fish inside. Firmly hold a cover over the top so fish cannot jump out, then slowly lower the pitcher into the new tank, but do not poor or rotate the pitcher; let each fish get its bearings and swim out of the pitcher on its own. Once you have the fish transferred over to the new aquarium, make sure that all of the components (heater, filter, lights) are working properly and that the water quality stays in the correct range. Then you can enjoy your bigger, better aquarium!