What Are the Dimensions of a Standard 20 Gallon Fish Tank?

The standard size of a 20-gallon fish tank is 30″ by 12″ by 12″ (long) or 24″ by 12″ by 16″ (high). The weight of this tank is 225 lbs.

Read on, to know more about if it suits your goal or not.

Ideal Dimension of a 20 Gallon Fish Tank

The ideal dimension of a 20-gallon fish tank when it is 20-gallon (high) is 24″ x 12″ x 16″ and when it is 20-gallon (long) is 30″ x 12″ x 12″.

It is a medium size fish tank often recommended to beginners. It is large enough to keep a good variety of fish. In the meantime, the volume of it is good enough to dilute the chemical reactions that happen inside it.

On the other hand, 20 gallons is not so big which is hard to handle and maintain. Which becomes a difficult chore for the new aquarists.

All those aside, fish tanks mostly depend on the fish you are planning to keep in them and how much space they need. So, before selecting the tank, selecting the fish is important.

Dimensions of a Standard 20 Gallon Fish Tank

How much Weight a 20 Gallon Fish Tank Holds?

Generally, the weight of the fish tank when empty is 25 lbs. and when filled with water is 225lbs.

If the gallon of the tank is determined already then, the weight will be specific. No matter what is the shape of the tank. every 20 gallons tank, shaped hexagons, pentagons, or even cylinders will weigh exactly the same.

In the case of a 20-gallon tank, as mentioned above. It is a moderate size, to begin with. 20 gallons is medium both in size and weight.

Size and Shape

The most common fish tank shape is rectangular. It is the shape in which, most of the fish can roam comfortably.

Besides, there is a variety of shapes and sizes of aquariums available in the market. As it said, rectangular is the most common one. Hexagons, bow-front rectangles, cylinders, etc. are also available but mostly custom-made. You have to give the order to make one for yourself.

When it comes to custom-made, acrylic aquarium is more easily available in different shapes and size. The shapes of acrylic aquariums are astonishing. You can go for this if you want an uncommonly designed aquarium to showcase uncommon fish. 

Does 20 Gallon Fish Tank Need a Mat?

If you are planning to get a rimless aquarium, then you should get a mat with it. but if your tank is braced then there is not much need for a mat.

Tanks get heavy when it’s fully set up. No matter if it is 20 or 40 gallons. Not only the water there will be sand, decors, filter, light, etc. All of them will weigh more than 225 lbs.

Yet, it will not be a problem if the surface where the tank will sit is fully leveled. But human-made surfaces can never be completely leveled. So, there is always a chance to get the tank cracked if it is rimless.

So, to keep safe, go for the mat. No matter if the thank is 20 gallons.

How Many Fish Can Accommodate in a 20 Gallon Tank?

It completely depends on what species of fish you want to keep in your tank. The rule you can follow is one gallon for an inch of every fish you put into it.

The rule is also vague. Because some species are big enough to fit two in the tank. But they need space to thrive in the aquarium.

There are some schooling fish as well. Say, you can keep 5 of them in a 20-gallon tank. But it is not sufficient for them. Because they will be needing more of their species to live happily.

You need to do your research before getting the fish you want. Research specifically every fish and their water and space requirement. You will have a good idea after that.

Some fish you can look for in your 20-gallon tank:

  1. Killifish
  2. Dwarf Cichlids
  3. Tiger Barbs
  4. Zebra Danio
  5. Mollies

How to Set Up a 20 Gallon Tank?

  • Firstly, place your aquarium on a flat surface, or on a mat if possible. It will make sure the tank has sat comfortably.
  • Clean out the substrate thoroughly because there can be dust which can completely ruin the water in the tank.

To clean it, put the substrate into a bucket and run water with a hose or even from a pipe. Wash it with your hands and let the water soak.

  • Now very gently put the substrate inside the tank. try to make a one-inch layer in the bottom of the tank.
  • Then place all the tank equipment but keep it unplugged and place all the decors. But make sure none of that touches the heater.
  • Do not add the live plants at this stage.
  • Start filling the water carefully. If you fill the water too fast, it will make a hole in the substrate which will create another mess.
  • When the tank is half filled, add the live plants and fill them with water.
  • Plug in all the equipment.
  • Place the lid and plug in the lights. Now it is time to cycle.

Frequently Asked Questions (FAQs)

Is a 20-gallon aquarium a good size?

Yes, it is. 20 gallons fall in the medium range which is good for beginners.

How often should you change the water in a 20-gallon fish tank?

You have to change 10 to 15 percent of the water every week.

What’s the biggest fish I can put in a 20-gallon?

You can keep fish that grow up to six inches max.

How many goldfish can live in a 20-gallon tank?

20 goldfish can live in a 20-gallon tank.


A 20-gallon tank is a good size if you are a beginner. You will get both the advantage of not-so-big as well as not-so-small. There are many varieties of fish you can look for to keep in a 20-gallon tank.

But it is always the best idea to look for the fish first. Look for low-maintenance and easy-to-keep fish. It will increase the chances of maintaining them in the long term.

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Arif H Fahim

Welcome to PetFishTank! My name is Arif. I make an effort to publish educational articles using my many years of expertise as a fish owner.

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