What size tank do I need for two goldfish?

When it comes to picking the right aquarium for your pet goldfish, bigger is always better. Goldfish are active swimmers that can grow quite large, so they need plenty of room to thrive. While many sources recommend a minimum of 20 gallons for the first goldfish, a tank of at least 30 gallons is ideal for two fancy goldfish. For common goldfish, which reach 12 inches or more at maturity, a much larger tank of 55 gallons or more is recommended.

What is the minimum tank size for two goldfish?

Most experts recommend providing at least 20 gallons for the first goldfish, and an additional 10 gallons per additional goldfish. So for two goldfish, the bare minimum tank size would be 30 gallons (20 for the first fish + 10 for the second fish). However, bigger is always preferable when it comes to goldfish tanks. These active fish have high bioloads and require strong filtration and regular water changes. More water volume helps dilute waste and provides cleaner, more stable water quality.

Why is a 30 gallon tank too small for two goldfish?

While a 30 gallon aquarium meets the minimum recommendation of 10 gallons per additional goldfish, it is still far too small to humanely house two fully grown goldfish long-term. Here are some reasons why:

  • Goldfish grow much larger than most first-time owners expect. Fancy varieties can reach 6-8 inches, while common feeder goldfish routinely hit 12 inches or more at maturity. Two adult goldfish will be cramped in a 30 gallon tank.
  • Goldfish are very messy fish with high bioloads. Their wastes can quickly accumulate and deteriorate water quality in smaller tanks, despite filtration. More water volume helps dilute waste.
  • Goldfish require plenty of swimming space to stay active and healthy. Insufficient space may cause stress, aggression, poor growth and more.
  • A 30 gallon tank is too small to create an ideal aquascaping environment for goldfish. It does not allow for ample open swimming space along with decor, plants and hiding spots.

While a 30 gallon tank may seem sufficient when the fish are small, it will quickly become cramped and unsuitable as the fish reach maturity. Investing in a larger tank from the start avoids having to upgrade later.

What size tank do two fancy goldfish need?

For two fancy goldfish, a tank of at least 55 gallons is recommended. Fancy varieties include fantails, orandas, black moors and ranchus. Here’s a breakdown of the ideal tank size:

  • Minimum of 20 gallons for the first goldfish
  • Add 15-20 gallons for the second fancy goldfish
  • Allow for adequate swimming space and aquascaping elements
  • Account for potential adult size of 6-8 inches each

With these factors in mind, a tank of 55 gallons or larger is ideal for two fancy goldfish. This allows them plenty of room to swim and grow to maturity. Make sure the tank is designed with goldfish in mind – it should be wider than tall to provide surface area for gas exchange.

Recommended tank setup for two fancy goldfish

  • Tank size: 55 gallons or larger
  • Filter: Canister filter rated for at least double the tank size
  • Heater: Not required, goldfish prefer water in 65-72F range
  • Substrate: Fine gravel or bare bottom
  • Plants: Anubias, Java Fern, Amazon Sword. Anchor to decor rather than substrate.
  • Decor: Driftwood, rocks, artificial plants. Leave ample swimming space.

What size tank do two common goldfish need?

Common goldfish require even larger aquariums than their fancy cousins due to their potential giant size. Provide the following tank sizes for two common goldfish:

  • Minimum 75-gallon tank for two juvenile common goldfish under 4 inches
  • Minimum 120-gallon tank for two young adult commons 4-8 inches
  • Minimum 180-gallon tank for two adult commons over 8 inches

Because commons grow so quickly and reach over 12 inches in length, they need very large tanks right from the start. Trying to upgrade tank sizes as they grow is difficult and stressful on the fish. That’s why it’s best to provide an adequately sized home based on their expected mature dimensions right from the beginning.

Recommended tank setup for two common goldfish

  • Tank size: Minimum 120 gallons
  • Filter: High-capacity canister filter rated for at least 200 gallons
  • Heater: Not required
  • Substrate: Fine gravel or bare bottom
  • Plants: Large Anubias, Java Fern, Amazon Sword. Anchor to decor not substrate.
  • Decor: Large driftwood, rocks. Allow plenty of turn space.

What factors determine goldfish tank size?

When selecting an aquarium for goldfish, there are several key factors to consider:

  • Adult size – Goldfish grow much larger than most people expect. Tank size should be based on maximum adult dimensions.
  • Swimming space – Goldfish are active swimmers and need long tank dimensions to accommodate their movement.
  • Waste production – Goldfish have a very high bioload. More water volume helps dilute waste.
  • Oxygenation – Surface area allows more gas exchange to maintain high oxygen levels.
  • Filtration – The filter system must be robust enough to handle the bioload of goldfish.
  • Behavior – Goldfish should have adequate space to exhibit natural behaviors and movement.

While tank dimensions are important, also pay attention to factors like water quality, swimming space, surface area and filtration capacity when selecting a new goldfish aquarium.

How many goldfish can fit in a 55 gallon tank?

Generally, a 55 gallon tank can comfortably house two to three fancy goldfish. This allows each fish adequate room to swim and reach maturity at 6-8 inches in length. Here are some 55 gallon stocking recommendations:

  • 2-3 fancy goldfish
  • 1-2 common/comet goldfish
  • 1 fancy goldfish + a few small peaceful community fish like tetras or Danios
  • 1-2 fast-moving plecos to help eat algae

When stocking any aquarium, it’s important not to overcrowd the tank. Water quality deteriorates rapidly in an overstocked tank. For goldfish, aim to provide 15-20 gallons of space per additional fish to keep waste levels low and oxygen levels high.

Tips for stocking a 55 gallon goldfish tank

  • Select young, small goldfish – this allows them room to reach adult size
  • Stick to just one or two common goldfish – they grow much larger than fancies
  • Add peaceful tankmates with low bioloads
  • Include fast-growing plants to help absorb excess nutrients
  • Use an extra powerful filter rated for at least 100 gallons
  • Perform 25-30% water changes weekly to maintain pristine water

Can you put two goldfish in a 20 gallon tank?

It is generally not recommended to house two goldfish in a 20 gallon aquarium. Here’s why:

  • 20 gallons does not provide adequate swimming room for one goldfish, let alone two.
  • Waste will rapidly accumulate and deteriorate water quality in such a small volume.
  • Ammonia and nitrite spikes are more likely in an undersized tank.
  • Stunted growth and other health issues are common when goldfish are cramped.
  • There is insufficient room for decorations, plants, hides – goldfish require enrichment.

While you may be able to keep two very small juvenile goldfish in a 20 gallon tank temporarily, they will quickly outgrow that space. Goldfish have high bioloads and should be provided with the largest aquarium possible. For long-term health and growth, each goldfish needs a minimum of 20-30 gallons.

Caring for goldfish in small tanks

If you must keep goldfish in a tank under 30 gallons temporarily, focus on the following to support their health:

  • Frequently test water parameters like ammonia, nitrite, nitrate
  • Change at least 50% of the water 2-3 times per week
  • Use extra filtration like sponge filters to supplement the main filter
  • Feed a high quality diet in small portions 2-3 times daily
  • Use an air stone to maintain oxygen saturation
  • Avoid tankmates and overcrowding

But again, a 20 gallon tank long-term is not suitable or humane for two adult goldfish. Upgrade them to a larger aquarium of 55+ gallons as soon as possible.


When it comes to selecting the ideal goldfish tank, bigger is always better. While a minimum of 20-30 gallons is often cited per fish, an aquarium of at least 55 gallons is recommended to humanely house and care for two goldfish long-term. Even larger tanks are needed for common varieties that reach 12+ inches in length. By investing in an appropriately sized tank, you ensure your goldfish have ample space to swim, grow and exhibit their natural behaviors for years to come.

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