What Size Tank Do I Need for Two Goldfish?


There’s a great deal of conflicting information online about common goldfish care. This can make it difficult for first-time fish owners to get reliable advice on food, equipment, and safe habitats for their pets. This lack of clarity is perhaps most evident when it comes to tank recommendations which seem to vary wildly with some experts saying bigger is better and others swearing ten gallons per fish is plenty.

It’s true that goldfish can survive, even thrive in fairly small tanks. You might have heard rumors of some shape-shifting species that grow to fit their environment, and this is true as well. Common goldfish do produce a growth-inhibiting hormone that limits their size when concentrated in small tanks.

A more important issue for the health of a goldfish is cleanliness. The smaller the tank, the quicker its water will fill up with feces, grime, and dangerous chemicals like ammonia. The smaller your fish’s tank, the more frequently its water must be emptied and replaced, and the more work you’ll need to do to protect his health.

Recommended Tank Sizes for Goldfish

The best tank for a goldfish is one that’s large enough to tolerate small amounts of water pollution between cleans. Unless you can change your goldfish’s water every other day, he needs a container he can safely eat and poop in without creating toxic concentrations of ammonia and other pollutants.

More room in the tank is also great for your goldfish’s brain and development. A bigger tank means more space for oxygenating, eco-boosting plants, water cleaning filters, and fun accessories to keep him stimulated. Yes, a pet goldfish can live in a small tank, but bigger is always better.

30 gallons for one common/comet goldfish

42 gallons for two common/comet goldfish

A good rule of thumb for tank size is to allocate 30 gallons of water to your first common or comet goldfish. Add 12 gallons for every additional fish placed in the water. Using these numbers as a guide, the recommended tank size for two goldfish is 42 gallons.

Fancy goldfish are a slightly smaller, less active goldfish species so they can be kept in a smaller tank.

20 gallons for one fancy goldfish

30 gallons for two fancy goldfish

Where possible, a goldfish’s tank should be a minimum of three feet in length. If the tank is without a filter, its water must be partially cycled every week. This involves removing and replacing a third of the tank’s dirty water with treated tap water once per week.

If you don’t treat tap water with a conditioner before adding it to the tank, its high levels of chlorine, chloramines and nitrates may negatively affect your fish’s health.

Caring for Multiple Goldfish

The tank sizes recommended here are exactly that: recommendations. You might find that multiple goldfish need even more space to live harmoniously in one tank. Contrary to popular belief, these fish aren’t stupid. Animal experts proved many years ago that common goldfish don’t have five-second memories. Like most creatures, they have distinct personalities and preferences.

As with any other type of pet, you should always match your care to the needs of your fish. Some goldfish hate to live alone, but others can be aggressive if forced to live too close to another tankmate. Depending on their personalities, two goldfish might only be happy living together in a tank that’s big enough for three or four fish.

The more goldfish you add to a tank, the quicker its water becomes polluted and eventually toxic. You can offset the need for weekly water cycling by using water filters to remove food waste and other impurities. The water still needs to be cycled and replaced (in thirds), but you can extend this to one cycle every 7 to 10 days if you add a filter to the tank.

Aggression between tank mates is common particularly when a new fish is introduced to an older pet. These unwanted behaviors should fizzle out in a few weeks after a pecking order has been established. If they do not and one or both goldfish continue to be aggressive, consider housing them in a bigger tank.

If this does not work, you may need to separate them. Like people, some goldfish are just grouchier than others!


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