What Size is Best for a Goldfish Tank?

Two goldfish in an aquarium tank

Khamla Douangchandeng/Getty Images

A pet goldfish is one of the most common species kept by beginner fish owners. However, what starts as a small fish can easily grow to a dinner-plate size behemoth. What tank size is best for goldfish and how should you plan accordingly for their large, adult size?

What size is best for a goldfish aquarium?

First, decide how many goldfish you want to keep. Goldfish do just fine as only children, so you can start with one, or keep two, three, or more. Keep in mind that the more fish you have, the more room they will need. When they are only a few inches long, you can get away with 20 gallons per fish. Yes, this rules out bowls, 5 gallon, and 10 gallon tanks.

Why so big? Goldfish are messy fish with large appetites and low feed to mass conversion. That means that for the amount of food they eat, they produce a lot of ammonia. Ammonia is the primary nitrogenous waste in fish and the beginning of the nitrogen cycle. Since goldfish produce a lot of ammonia, having more water to dilute this out makes a big difference in the overall health of your goldfish. If ammonia builds up in the water it can stress or even kill the fish.

And a 20-gallon tank is just to start! As the fish grow, expect to upgrade to a 50, 75, or even 100-gallon tank for one goldfish. Standard comet goldfish can grow up to 16 inches long—the same size as a dinner plate—and they will need lots of room to swim. Fancy goldfish varieties do not get as large and if they have difficulty swimming, may not need such a dramatic tank upgrade.

Goldfish in 100 gallon tank
This goldfish was won at a carnival over 16 years ago and has his own 100 gallon tank in an accounting firm.

 Jessie Sanders

What do goldfish need in their aquarium

Goldfish require a large swimming space and lots of filtration. If you have a standard, long-body goldfish, such as a comet, sarasa or shubunkin, your filtration should be slightly larger than what is recommended for the tank size. Remember, goldfish produce a lot of waste, so excess filtration is a great benefit. However, many fancy varieties of goldfish are not great swimmers, so excessive currents from strong filters can easily cause them stress. Baffling or redirecting filter outflows to the longest angle of your tank is best at providing adequate filtration without excessive water currents.

Your tank should be long enough for your fish to swim comfortably and turn around without difficulty. You can have artificial decorations, provided they do not impede your fish's ability to swim or turn around. Keep in mind that any caves may be quickly outgrown as your fish grows.

You can keep live plants with goldfish, but be sure to quarantine them properly. If they come from aquarium systems with other fish, they could also potentially bring in bacteria and parasites. A two-week quarantine in a tank containing no fish will break parasite life cycles. Use this time to beef up your new plants with a bit of aquarium plant fertilizer. Don't get too attached to your plants, however, since goldfish are notorious redecorators and plant eaters.

How do you know if your goldfish needs a bigger tank?

Goldfish do not grow only to the size of their container. Terrible water quality and ancient flake diets have perpetuated this myth for far too long. In an appropriate environment and with a good quality diet, expect your fish to reach full size within a few years.

The most prevalent indicator of a goldfish that has outgrown the tank is poor water quality. You may see nitrite or ammonia in an established aquarium, indicating that your filtration cannot keep up with your fish's waste production. You can add more filtration temporarily, but a bigger tank is a much better permanent fix.

Another sign of a goldfish running out of room is difficulty turning around. Some fish may actually develop spinal curvature from small tanks. In shallow tanks, a fish's tail hitting the bottom during feeding time is another sign that it's time for a larger tank. Thankfully, once you move the fish to a new, bigger tank, the spinal issues will likely resolve, as long as the fish has not been in the old tank for too long.

Bruises and missing scales are another sign that your fish is running out of room. In the interim, before you purchase a new tank, you can buy some time by removing décor items from the tank and giving your fish more room to swim.

Tips to Maintain Goldfish at Current Size

In order to preserve your fish at their current size in their current tank, it is critical for you to pay special attention to their diet. Diets high in protein will cause your fish to grow big faster than if they were on a lower protein diet. Overfeeding a low protein diet can also lead to rapid growth.

The best feeding practice for goldfish is to feed a pelleted diet between 30-35% protein. Depending on the water temperature, your fish will require once or twice a day feeding. If the temperature is below 74F (23C), feed your fish once a day. Above this, feed twice a day. The warmer the water is, the faster your fish's metabolism and the more calories they will burn swimming around.

During feeding time, feed your fish a few pellets at a time. Allow them to consume all the pellets before adding a few more. Most goldfish will slow down or stop eating after a few rounds of light feeding or a few minutes. After this, stop feeding. Always watch your fish carefully during feeding rather than dumping the food in and walking away. Feeding time is a great way to check out your fish's behavior and appetite—a good time to assess if anything may be wrong.