How To Set Up A 20 Gallon Fish Tank Or Aquarium

The 20-Gallon Fish Tank Or Aquarium

How To Set Up A 20 Gallon Fish Tank Or Aquarium12 mins read

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Tal Halperin
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The 20-Gallon Fish Tank Or Aquarium
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If you want to set up a 20 gallon fish tank, this is a great choice for aquarists of all levels. A 20 gallon tank is easy to set up, does not take too much maintenance, and also doesn’t cost too much.

This article will demonstrate the best fish for a 20 gallon tank, ideal 20 gallon fish tank dimensions, and how to set things up properly to save trouble down the line. This is an ideal project even for those at the beginner level. We go into basics like ‘how many fish in a 20 gallon tank is ideal’ as well as information for intermediate and advanced aquarists too.

I set up my own tank to see how the process worked firsthand. In addition, I spoke to professional aquarists at specialist aquatic stores to gather the best information on setup, plants, tankmates, and more. So, read on to find out!

Understanding The Basics

What things do you need to know before setting up a tank? Generally, it’s ideal to get an overview of the needs of the project. This will prevent problems down the line.

Aquarium Size and Space Considerations

Before you buy your tank, think about where you want it to go. A 20-gallon fish tank may not look like it takes up much room. However, once you have factored in wiring and the space you need to store equipment like your tank vacuum, this can increase. For inside dimensions, ‘how many fish can you keep in a 20 gallon tank’ is not a question with a set answer. However, do think about whether you want length vs height. More active fish species tend to need a long fish tank.

Types of Freshwater Aquariums

You can either often find complete 20 gallon fish tank kit, or you can buy the components separately. Tanks come in many shapes and sizes, as well. Many people want a 20 gallon long fish tank for fish that like lots of swimming space like barbs. These tanks also give you the chance to see your fish very easily. Alternatively, square, vertical, or hexagonal fish tanks exist. Either way, you may need to add on a specialist fish tank stand too.

Benefits of a Twenty-Gallon Tank

A twenty gallon tank is useful for those who want to keep fish but don’t have a lot of space. It’s also ideal if you are on a budget, or don’t have much time for tank cleaning and maintenance.

Furthermore, a 20 gallon tank setup is very often the first one for beginners – and for good reason. So, let’s get started…

Essential Equipment

Before setting up a tank, always get your equipment together first. Remember, it’s quite hard to stop as you go along when you set up a fish tank. Therefore, you should ensure you have all the equipment ready before you begin the setup process.

1. Tank and Stand

Lifegard Aquatics 20 Gallon Rimless Clear Glass Aquarium

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To decide which tank and stand to use, you first need to choose the location of your aquarium. If you want to place the aquarium on a desk or other flat surface, you won’t need a stand. But if you want to put it elsewhere, you will have to factor a stand into your budget. 20 gallon long fish tank set ups are one of the easier options but take up more surface area.

A 20 gallon fish tank stand should be sturdy and stable. It shouldn’t wobble and should be made of a strong material like wood or metal to withstand the weight of the tank and the water once your aquarium is full.

Lastly, the stand should be more or less larger than the base of the aquarium, so the aquarium does not fall off.

In terms of tanks, there are many you can choose from. Even at this size, fish tank dimensions are not all the same.  Generally, a square glass tank that is wider than it is tall, or at least cuboid, is best. 20 gallon hexagonal fish tank setups can work, but generally only for small schooling fish.

This is because it gives your fish plenty of opportunities to swim from side to side and allows you to watch them from multiple angles. A greater width also means you have more surface area on the bottom of the tank to creatively sculpt many areas for your fish to hang out in.

Many people enjoy modern-looking, rimless tanks. They’re a great choice that doesn’t distract from the appearance of your fish, too. We recommend this 20 Gallon Rimless Aquarium from Lifeguard Aquatics.

2. Filtration System

quaClear 20 Power Filter

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There are multiple types of aquarium filters. Fortunately, however, there are only a couple that are suitable for the kind of aquarium we are setting up here. Remember, your tank cannot go without a filter! However, you will still need to know the difference when purchasing online or at an aquatics store. Some of the most common are as follows:

  • Hang-on-back filter

A hang-on-back filter does exactly what the name suggests and clips onto the back of the tank so that the majority of the filter weight is outside the tank. It gathers water from inside the tank with a tube attached to a small unit that goes inside the tank itself. Many aquarists like this as it keeps the back wall of the tank free and therefore nicer to look at.

  • Sponge filter

A sponge filter is exactly what it says on the tin – a filter with a sponge that catches debris and detritus and sits in a compartment in the back of the tank, or one that hangs on the back.

The benefits of a sponge filter are they are gentle and therefore won’t hurt delicate, slow-swimming fish. Generally, a 20 gallon fish tank filter will be a sponge as opposed to canister filter. This includes many popular fish for small aquariums, like betta fish.

One con is that they normally fit into a compartment at the back of the tank. Therefore, you will have to ensure your aquarium has this. If not, you can get hang-on-back sponge filters.

  • Canister filter

Confusingly, a canister filter can also hang on the back. This refers to the type of filtration, not necessarily where you keep the filter in the tank.

A canister filter is quite strong and uses one pump to suck in water and one pump to push it out again. In between them, within the filter container, there is a chemical medium like activated charcoal that filters the water.

A pro is these filters are quite strong and therefore they are ideal for bigger tanks. However, a con of them is that they do not have a sponge to build up the beneficial bacteria. For a 20 gallon aquarium, a sponge filter is better than a canister filter. If you’re stuck as to which filter to get, we recommend this Aqua Clear 20 Power Filter, suitable for 5 to 20 gallon aquariums.

3. Lighting

Lighting is easier than a filter to choose. It’s also not essential, so if your tank is in an area with lots of natural daylight, you may not need it.

However, one of the best types of lights is an LED light. These often clip onto the back wall of your tank, or they come provided with the tank. However, some species of small fish for 20 gallon tank setups, such as the cardinal tetra, need low lighting.

With these fish, you can either opt for a dim bulb (available online) or you can add floating plants to the surface of your aquarium to shield your fish from the light.

Author’s Note: Remember that whatever light you choose, the electrical components should be behind glass. This prevents any water touches them and causing dangerous shocks.

4. Heater

Fluval M 200 Watt Submersible Heater

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A basic heater is essential if you are going to keep any ornamental tropical fish at all. You can get ones that attach via suction cups to the inside of the tank. They will heat the water to a set temperature, which you will be able to find marked on them somewhere.

Make sure you check what species of fish you are going to have and find an ideal average temperature for all the species! Remember also to check if your tank is near a radiator or open window that could affect the heat of the water inside. Anything too extreme, and it may be best to reconsider your aquarium location. We recommend this 200 W Submersible Heater from Fluval.

5. Thermometer and Other Tools

Lastly, there are some optional extras you can add to your tank. The two most common are a tank thermometer, which attaches to the side and will tell you the water temperature, and a bubbler. A bubbler is a small divide that sits under the gravel and creates a stream of oxygen bubbles. This oxygenates the water, making life easier for your fish.

Setting Up Your Tank

It’s best to choose a sturdy, flat surface that has space around it, such as a desk or tabletop. Sydney Perry /

1. Placement and Initial Setup

It’s best to choose a sturdy, flat surface that has space around it, such as a desk or tabletop.

Additionally, consider whether you want to keep plants. If you place your tank in direct daylight, this can help them grow. Conversely, if your tank is near a radiator or open window, it can become too hot or too cold. The best option is a spot that is away from extreme temperatures and has moderate daylight. In addition, cold temperatures can reduce the beneficial bacteria in your tank.

2. Substrate

The substrate you use in a 20 gallon tank will vary based on the environment you want to create. Do you want an Amazonian tank setup with a lot of plants? To help them take root effectively, you may need aquarium soil. Do your fish need a pH that is on the higher side? Some, such as betta fish, don’t like acid conditions. Therefore substrates like lava soil or crushed coral can help raise the pH.

In addition, consider how much room you have from the top to the bottom of the tank. A 20 gallon long fish tank has less vertical space. Therefore if you have many layers of substrate, you will have less room for your fish to swim.

However, if your tank is taller, some aquarists like to layer substrates. The most common combination is some sort of aquarium soil such as lava soil or scaper soil. Then, a layer of gravel or sand goes on top for aesthetic purposes.

Remember, if you’re stuck, you can’t go wrong with basic aquarium soil, a few live plants, and some gravel.

3. Adding Water

Before you add water, it’s important to ensure you don’t need to change anything about your tank. Sydney Perry /

Now for the tricky bit! Before you add water, it’s important to ensure you don’t need to change anything about your tank. As we mention above, it’s much harder to change things about your tank once it is full. Look around. Are all the wires in the right place? Do you have residual current devices? Is there a risk of water going on your electrical equipment? Are you 100% happy with the location of your tank? If you’re not, now is the chance to move it.

If the answer to all of this is yes, you are ready to add water. It’s best to add water in two stages. First, you can fill the tank halfway. Then, you can install your equipment such as the filter, heater, and even a bubbler if you desire.

After this, fill the tank to the top. Remember to adhere to any specific instructions for your filter or heater. For example, pay attention if it says on the packaging that the unit should not be fully submerged.

4. Installing Equipment

Installing equipment is relatively simple, but there is one golden rule. Make sure you install your equipment before you add water to the tank and that you are happy with it!

Secondly, note that 20 gallon fish tank dimensions listed on the product description may not take into account a compartment at the back for the filter and heater. Your equipment may take up some of your actual tank capacity! Therefore, provided you have access to a brick-and-mortar aquatics center, check in store beforehand.

Furthermore, it’s very difficult to adjust or modify equipment after you have filled your tank. It can be awkward to reach into the tank and some equipment may sit in a cramped compartment that is difficult to access. This is doubly so with water in the tank.

To test that everything is working, some fish keepers like to fill their tank halfway, install and check the equipment, and then fill the tank all the way to the top too.

Author’s Note: Note that 20 gallon fish tank lid types can vary and don’t always come with the tank. It’s best to check beforehand, and, if you are keeping fish such as barbs that like to jump, to invest in a lid.

Cycling The Aquarium

You may think that you are now ready to add fish However, just because your tank is full of water, plants, and other exciting things you have put there, doesn’t mean it is a good environment for fish. First, you need to cycle your aquarium to ensure the water quality is optimum.

All About The Nitrogen Cycle

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What is the nitrogen cycle? The nitrogen cycle is an invisible process that goes on in every fish tank. It’s the manner in which bacteria break down harmful ammonia, a chemical found in fish waste, into less harmful nitrates and nitrites. These less harmful chemicals actually help fertilize your plants!

Nitrates and nitrites are not totally harmless, however. Most fish can tolerate levels of both of these of 20 ppm. However, fish can tolerate levels of 0ppm for ammonia. Therefore if your nitrate and nitrite levels are a little high, don’t worry, but it’s crucial to ensure your ammonia doesn’t spike. In a tank with a fully functioning nitrogen cycle, the levels of ALL of these should be at 0ppm. Many fish for 20 gallon tank environments are quite sensitive, like the betta.

Author’s Note: Did you know all kinds of beneficial bacteria will live in a well-looked-after aquarium? These even include different bacteria in different parts of the tank, such as on the plants, in the substrate, and elsewhere.

Cycling Methods

Your tank should already have a filter, and this goes a long way to setting up the nitrogen cycle. However, to kickstart these biological processes, it’s best to add some beneficial bacteria to the tank. You can purchase this online or from any major aquatic center. It helps start the natural biological processes that maintain optimum water quality. Good bacteria also need the chance to colonize the filter, which is where they break down many harmful waste products.

In addition to adding bacteria, it’s always good to have some live plants in the tank.

While it’s possible to have a tank without live plants, it does make a difference and helps your tank cycle more smoothly, leading to a cleaner environment for your fish.

Author’s Note: In addition, there are products you can buy to optimize your water quality and make it safe for fish. We recommend Easy Balance Plus – if you live in an area with especially bad water quality we would suggest using this.

Testing The Water

You can buy water testing kits online or likewise from any aquatics store that also sells fish. Sydney Perry /

Is your tap water chlorinated? If you live in the US, the answer is definitely yes. However, this isn’t great for most fish species. As a result, you’ll need to ensure the water you add to your tank has been filtered. Alternatively, if you put tap water directly into your tank, you can add a dechlorinator chemical. Like the beneficial bacteria, you can purchase this online or from an aquatics store.

Once your tank has cycled for a month, it’s time to test the water. Some people recommend only cycling for a week. However, a month is ideal, especially if you intend to keep sensitive species like betta fish.

How to Choose a Kit to Test Your Fish Tank Water

You can buy water testing kits online or likewise from any aquatics store that also sells fish. You will have to buy tests for ammonia, nitrates, nitrites, and pH. Many stores stock all-in-one tests, but it’s best to check on the back of the box first. These kits also come with full instructions on how to use them and read the results. However, they generally involve adding chemicals to a small section of your tank water that you have taken out of the tank beforehand and put in a separate container. You won’t need to buy any separate equipment, as everything is included in the kit.

Ideal Water Parameters

Ideally, ammonia, nitrates, and nitrites should be at 0ppm before you add your fish. An ideal pH (indicated by color – most test kits have a guide so you can compare the color of your water to different readings) is between 6.5 and 7.5 for most warm-water freshwater fish. However, for some individual species, these may be different, so learn about the requirements of your species first.

Adding Fish And Plants

1. Planting Your Aquarium

When planting your aquarium, think first about what fish you plan to include. There are two things to consider when planting – what to plant, and where to plant.

If you have shy fish that need the cover of plants, think about thickly planting around the edges of your aquarium with some clumps in the middle.

If you have bolder fish you can get away with just plants around the edges.

Try to have a mixture of sizes, with small plants at the front of the tank, medium ones in the middle, and taller ones at the back or near any ornaments you may have.

There are nitrogen-rich fertilizers available for aquatic plants that you can add to the water and or soil. However, if the nitrogen cycle in your tank is going well, you shouldn’t need to.

In terms of what to plant, there are a few options. Bearing in mind the sizes of plants and creating a habitat for fish, here are some to consider:

Tall Plants

  • Amazon sword
  • Madagascar laceleaf
  • Cabomba
  • Water wisteria

Medium Size Plants

  • Java fern
  • Anubias
  • Water hyssop
  • Gratiola species

Small Plants

  • Guppy grass
  • Java moss
  • Marimo moss ball
  • Baby tears
  • cryptocoryne

Additionally, you can add floating plants to your aquarium. These can help provide shade for fish that are light sensitive, as well as add a sense of naturalism to your aquarium. Good floating plants include:

  • Water wisteria
  • Mosquito fern
  • Amazon frogbit

2. Choosing Fish

Betta fish are amongst some of the best fish for a tank of this size. Sydney Perry /

Now it’s time to choose your fish! So, how many fish in a 20 gallon tank is ideal? You may like the look of some fish such as angelfish but these require a much bigger tank. It’s important to check both the species and quantity you want are suitable for a tank of this size. In addition, some species require a group with a minimum number of tankmates so you will have to factor this in too.

Some of the best fish for a 20 gallon tank include the following:

  • Betta fish
  • Gouramis
  • Pleco fish
  • Cory catfish
  • Dojo loaches
  • Oto catfish
  • Various types of tetras

3. Introducing Fish To The Tank

You will have to acclimatize your fish to the water temperature and quality first. Sydney Perry /

Once you have taken your fish home, it’s important not to just put them straight in the tank. Instead, you will have to acclimatize them to the water temperature and quality first. You can do this by opening the bag your fish came in and gently floating it in the water. This will allow the temperature inside the bag to reach equilibrium with the temperature outside. This will prevent your new fish from getting temperature shock.

Maintenance And Troubleshooting

1. Regular Maintenance Tasks

How to change the water in your tank:

The biggest regular maintenance task you will have to undertake in your aquarium is changing the tank water. This is best to do once every week and ensures that your water parameters remain ideal and your water stays crystal clear.

You can read more about what this involves and the specific equipment you need here in our guide on how to clean a betta fish tank.

How to clean algae off your tank’s glass:

In addition to water changes, algae buildup can be a problem in many aquariums. You may have algae-eating fish, but even in this case, the amount sometimes gets too much for them.  Therefore you may have to use an algae scraper.

How to change the water media in your fish tank: 

Lastly, a less frequent maintenance task you will have to undertake is to change the filter media. This is something you should do every three months and requires you to purchase new filter media from the fish store first. Then, you simply need to replace the old filter media with the one you purchased.

What you can also do once a month is wash the filter sponge. Don’t do this with soap and water! You’ll kill the good bacteria responsible for the nitrogen cycle in your tank. Instead, wait until you are doing your weekly water change, and wash it in old tank water before replacing it again.

2. Common Issues And Solutions

Even if you have done everything correctly, problems can still occur. Fortunately, this is simply part of having an aquarium and does not necessarily mean you are not a good fish keeper! Instead, these common problems have simple solutions.

Algae Growth

Algae growth can happen if your tank gets either too much or not enough light. It also happens naturally simply because algae is part of the tank ecosystem. However, it looks unsightly and can through the delicate nitrogen cycle out of balance.

To fix this problem, you can use an algae scraper to scrape the algae off the sides of the glass. This is a tool that consists of a flat rubber scraper on the end of a metal or plastic pole that allows you to work on the inside of the tank walls.

Algae eating fish like Pleco fish can clean algae from the walls of your tank. Sydney Perry /

However, if you want to save time, you can invest in some algae-eating fish or invertebrates. These include:

  • Pleco fish
  • Oto catfish
  • Cory catfish
  • Malaysian trumpet snails
  • Apple snails
  • Mystery snails

Cloudy Water

Cloudy water is generally a sign that there are too many pollutants in the water. Pollutants can take the form of excess algae that has come off the taken walls, as well as fish waste, and uneaten food.

These pollutants can cause high concentrations of ammonia and nitrates. Therefore, if you see cloudy water it’s important to clear it up as quickly as possible. Very often, it is a sign your filter is no longer working or you need to change the filter media.

Whenever you see cloudy water, you should always give your tank a water change, too. This is easy to do; simply follow the same steps as you would for a regular weekly water change as laid out in the maintenance section.

Dying Plants

Lastly, dying plants can be an unsightly and frustrating sight in your tank. It can also be a sign that the tank ecosystem isn’t working correctly. Dying plants can have multiple causes depending on how they look. These are the main ones to look out for:

  • Yellow leavesyellow leaves are a classic sign of nutrient deficiency in plants. Some of the most common deficiencies, all of which can result in yellow leaves, are nitrogen, potassium, and phosphorus deficiencies. You can solve these by adding some fertilizer to your plants.
  • Brown leaves – brown leaves are a sign your plants may not get enough light. They can also indicate dirty water, or that your plants can’t properly absorb nutrients from the soil. Check they are planted correctly, and change the tank water if possible.
  • Slimy plants – this can often come from poor water quality. One thing to note is that your plants may be even more sensitive than your fish. Therefore if your plants turn slimy, take note, and test the levels of pollutants in your water. You can simply go straight ahead and change the tank water, too.

3. Observing Fish Health

There are a number of health problems that fish can fall prey to, but these mostly come from dirty water. As a result, the health of your fish and the quality of your water are directly linked.

There are some general signs of fish illness that you should look out for, as well as specific diseases. The general signs are:

  • Lethargy
  • Fish stops eating
  • Fish hangs around near the bottom of the tank
  • Cloudy eyes
  • Bloating
  • Discolored faeces
  • Inability to stay afloat
  • Gulping or gasping for breath

Furthermore, these specific diseases are some of the most common that you may (but hopefully won’t encounter.

  • Fin Rot
  • Swim Bladder Disease
  • Indigestion
  • Dropsy
  • Velvet or Gold Dust Disease
  • Hexamita

You can read about them in more detail, as well as how to spot them, in most of our fish care guides. However, all of these are caused by dirty water to a greater or lesser extent. Therefore, so long as you change your tank water weekly, you should be able to prevent them.


It may seem complex to set up a 20-gallon aquarium, however, it’s not too difficult when you break it down. There are multiple stages and most aquarists find it helpful to have everything prepared first.

Remember, even if you’re a beginner, everyone has to start somewhere! An aquarium of this size is the perfect way to get the hang of fishkeeping and provided you take it slowly you will find that you can easily set up an impressive tank.

Frequently Asked Questions

How much does it cost to set up a 20-gallon fish tank?
It’s hard to say, as tanks vary in price, and getting an all-in-one kit can lower the cost of your setup. However, expect anything from around 250 to 500 or even 600 USD to set up a basic 20 gallon aquarium. It may sound like a lot, but investing in a high quality aquarium will ensure you don’t run into problems down the line. This cost estimate also factors in your filter, plants, substrate, and other bits and pieces like fish food.
How many fish can you keep in a 20-gallon tank?
In total, how many fish can be in a 20 gallon tank? The answer to this question depends a lot on the size and species of fish you choose. However, a general rule of thumb is anything from 3 to 6 is ideal. This might involve a school of all small fish or a larger (but still relatively small) fish like a betta and some tankmates like cory catfish. As a rule, the smaller the fish, the more you can keep - just make sure they get on and give them space to grow!
Is a 20-gallon aquarium suitable for beginners?
Yes, definitely! In fact, a 20 gallon aquarium is one of the most suitable for beginner aquarists. This is because many of the fish species that you can keep in it are easy to take care of. In addition, it’s small enough that it doesn’t require a huge amount of maintenance. You may even be able to find a 20 gallon fish tank kit that has all parts included.
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Sydney Perry

Sydney Perry has loved fish since she was a child and has enjoyed keeping many varieties over the years, ranging from black moors and shubunkins to betta fish. As a lover of nature and of Japanese culture, her dream tank is an Iwagumi aquascape, combining fish with carefully crafted aquatic landscapes in miniature.