Home Aquariums How Many Goldfish In A 20 Gallon Tank?

How Many Goldfish In A 20 Gallon Tank?

by Lucy

Goldfish can make such fascinating pets and anyone who has the pleasure of owning them knows how therapeutic a tank full of goldfish can be. Those that are new to fish ownership often have a ton of questions related to housing and care which is understandable.

One of the questions we most commonly hear is how many goldfish can be housed in a 20-gallon tank. This is a very valid question.

So, how many goldfish in a 20-gallon tank? The general rule of thumb is 1 gallon of water per 1 inch of fish. This means a 20-gallon tank is big enough to house 20 small goldfish. It is important for the health and wellbeing of your goldfish that you provide them with adequate space to swim freely.

Goldfish need enough space to be able to swim around each other without constantly clashing with other fish. They need the ability to swim away freely in their own space. Overcrowding in the fish tank can often cause severe health problems for the fish in question or in some cases even death.

Let’s take a closer look at the implications of an overcrowded fish tank and how you might help prevent such an issue from occurring.

What size tank to buy for my goldfish?

Buying a tank for your goldfish does not have to be a complicated procedure. Some believe that goldfish only grow to the size of their tanks whilst others are adamant that this is simply just not true.

Either way, we believe it is important that you provide your goldfish with enough space to swim around freely without constantly coming into contact with other fish in the tank.

Goldfish Tank Sizing Chart

1 – 10 Goldfish

5 – 10 Gallon Tank 

10 – 20 Goldfish

10 – 20 Gallon Tank

25 Goldfish

25 Gallon Tank

30 Goldfish

30 Gallon Tank

40 Goldfish

40 Gallon Tank

Although goldfish are freshwater fish and do not require a heater (tank temp. should be between 50-70°F) they do tend to create lots of waste so a strong filter is highly recommended.

Try to stay away from tall or novelty tanks if you can. Rectangular shape tanks are usually best for goldfish.

Make sure that you make allowances for items placed in the tank such as rocks, plants, and other novelty decorations. This will take up valuable space and where you might have 20 fish in a 20-gallon tank they may in theory have only 10-15 gallons of space once the decorations are in place.

How do I know if my goldfish tank is overcrowded?

Sticking to the rule of 1 inch of fish per 1 gallon of water is a very good guideline to follow for the health and wellbeing of your goldfish.

However, there are also some key things to specifically look out for if you feel like your fish tank may be overcrowded. These signs will give you a very good indication that you have too many goldfish in the tank together.

Low oxygen levels

One of the key signs your fish tank may be overcrowded is by looking at the oxygen levels. This is not easy to determine and requires you to really study your fish’s behavior and patterns. Low oxygen levels can be a sign that you have too many fish in one space.

Look out for some of these keys signs of low oxygen levels:

  • Lack of swimming and eating
  • Heavy breathing – this will be visible through the grill movements
  • Gasping at the surface for air

Ammonia in high levels

High ammonia levels can be another key sign that you have too many fish in one tank. Ammonia levels should be taken seriously as this compound is highly toxic to fish and could cause death.

The reason fish tanks are so prone to high ammonia levels is thanks to the increased levels of waste goldfish produce. It is created by the decomposition of waste and even small concentration levels can cause huge problems.

The easiest way to tackle this problem is by ensuring you have a decent water filter. However, if your tank is overcrowded the ammonia will not get neutralized very easily by the beneficial bacteria quick enough which will cause your goldfish issues.

High nitrate levels

Nitrate levels in your goldfish tank work hand in hand with ammonia and it is created when the beneficial bacteria are decomposed. Although in low to medium levels nitrate is not too much of an issue, if high concentration levels are present in your tank then this is likely to cause some serious problems for your goldfish.

Often, high nitrate levels can be a result of too many fish in one tank. This can lead to bacterial and fungal infections.

The easiest way to combat this is by making sure you have a good filter system and that the water in the tank is changed regularly. Adding live plants that will feed off the nitrates is also a very good solution to this problem.


As you can imagine an overcrowded goldfish tank can cause some behavioral problems. Too many goldfish in one space can result in aggressive behavior towards one another. It can also result in some fish becoming highly territorial.

Giving your goldfish enough space to move around with ease leads to a much calmer, peaceful life in the aquarium setting. Fights that break out due to lack of space can result in injuries such as fin nipping. This in turn could lead to unnecessary stress and infections.

Stunted Growth

Some aquarists believe that an overpopulated tank can stunt the growth of goldfish. It is believed that fish release growth-suppressive hormones that allows them to adapt to their environment. This is known to ultimately stunt their growth if they do not have the means to do so.

This is a highly debated theory. Some believe this is true however others believe this is nothing but a myth.

What type of 20-gallon tank should I buy?

There are many types of goldfish tanks on the market today and the designs have evolved massively to suit the everyday needs of fish owners and aquarists.

What to look for when shopping for a 20-gallon tank:

  • Always make sure you buy a tank big enough for the amount of fish you would like to house
  • Allow space for tank decorations, plants, etc. This space consideration should always be factored into your tank size calculations
  • A standard, rectangular shape tank is always best
  • Make sure the tank you are buying is sturdy, durable, and well-sealed
  • Look out for a good quality filter with good lighting

20-Gallon Goldfish Tanks – Product Reviews

Below are 3 fish tanks that work great for housing up to 20 goldfish.

1. Tetra 20 Gallon Complete Aquarium Kit

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The Tetra 20 Gallon Complete Aquarium kit is quite possibly one of the best goldfish tanks on the market at the moment.

This kit provides you with just about everything you need to set up a fully functioning aquarium.

With a stylish modern-day feel, the Tetra 20 Gallon Complete Aquarium kit is sure to be a central focus for any home.

Key features

  • Color changing LED lights with fully customizable features
  • Scratch-resistant glass and durable materials
  • Tetra plants included, that come to life when switching on the LED’s
  • Full kit including a UL heater, hood and Tetra Whisper filter


  • A complete kit to get you fully started for ultimate convenience
  • LED lights that often need to be bought as an additional add on
  • Made in the USA with a lifetime guarantee for complete peace of mind.
  • Good value for money with some top customer reviews!

2. SeaClear Acrylic Aquarium

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The SeaClear Acrylic Aquarium is a basic, but a modern looking tank that aesthetically has the potential to fit in well inside any home environment.

This tank has been designed by the internationally recognized SeaClear brand and the reviews it receives pretty much says it all!

Key features

  • Made from acrylic glass that is durable and 17 times stronger than regular glass
  • Reflectors and 15” light fixture included
  • Basic design, that is stylish and blends in well with a variety of rooms
  • Impact-resistant and less prone to chipping and scratching


  • A basic design, that is stylish and blends in well with a variety of rooms
  • Safe for children and pets to be around thanks to the acrylic glass that will not crack or shatter
  • Lightweight and super easy to move around (weighs half the weight of regular glass tanks)
  • Not the cheapest on our list, but still very good value for money

3. Lifegard Aquatics Crystal Aquarium

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The Lifegard Aquatics Crystal Aquarium is a popular brand with a great reputation for providing some of the best fish tanks on the market.

It is sold in three different sizes housing up to 24 fish at a time giving you the ultimate choice of sizing.

Key features

  • Ultra-low iron glass producing a much clearer view
  • Built-in back filter – which often needs to be purchased separately at an additional cost with other brands
  • Invisible glue joints to keep the tank completely sealed with no chance of leaking


  • Crystal clear 45° edge rimless tank for the ultimate viewing experience
  • Three different models and sizes available to suit all needs
  • Great value for money whilst still maintaining quality.

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