The Best 20 Gallon Fish Tanks

If you’re looking into getting a new 20 UK gallon tank (25 US), whether you are new to fishkeeping or have had aquariums for years, you may be bombarded with choice.

What do I need for a 20 gallon? Which are the best tanks on the market? What equipment will I need for a new 20 gallon tank? 

This and more, we will aim to answer in this article.

Is a 20 gallon fish tank a good tank size?

For goldfish, a 20 gallon tank is alright for a time. It will be able to house very young goldfish of up to 3” for maybe a year or so, but they will outgrow it and it’ll need an upgrade eventually.

In general, a 20 gallon tank is an excellent starter tank – we much prefer them over 10 gallons, as they give for a lot more variety and are more forgiving of mistakes and neglect as they can hold more water.

A 20 gallon tank gives you more variety in the fish you can choose to keep, as the extra space is much less limiting than say a 10 gallon, which only allows you to keep a few very small fish.

20 gallons are easy to manage and are a great aquarium size to start off with.

What do I need for a 20 gallon fish tank?

Most aquariums these days are sold as kits, containing most of the things you need. Some however, don’t come with everything you need and may be missing some parts.

Here, we will go over all the equipment you will need on a 20 gallon to run it successfully for the fish you want to keep.

For a 20 gallon fish tank, you will need a filter that is sufficient enough for the tank. You are looking for something that is able to filter over 100 litres; most filters will say on the box what they can do and often come in different sizes.

Depending on the type of tank you want to go for, you will also need an LED aquarium light to turn on during the day, so you can see your fish.

Adding a light isnt fully necessary however, unless you have live plants, in which case they will need light to feed and grow from.

If you are going for a tropical tank, then you will also need a 100 watt glass aquarium heater, so you can keep it at the ideal temperature.

You may also want some tank decor, perhaps a sand or gravel of your choice to add to the bottom, some plants and perhaps some rocks or driftwood.

Should I get a 20 gallon fish tank?

20 gallon tanks are an excellent sized aquarium. They are reasonably sized, while not taking up much room,and hold a good amount of water, which means you can house a larger variety of fish, giving you more choice in what you want to keep.

They are easy to maintain as the large amount of water means problems will happen at a much slower rate, meaning you have more time to react to things like poor water quality.

20 gallons gives enough space for a comfortable community tank; something which smaller tanks cannot do so easily.

20 gallon tanks are also typically longer than most aquarium sizes, which means they will be much cheaper than a tall tank, as the glass does not need to be as thick.

They are however, still not large enough to house a goldfish for its whole life, although they can house a few young goldies for a time.

20 gallons are best used to house smaller tropical or coldwater fish like guppies, tetras and danios, but are an excellent starter tank all in all.

What to look for in a 20 gallon fish tank

When choosing a 20 gallon tank, you want to look for an aquarium that is well dimensioned. By this, we mean that the sizes width, depth and height are all reasonably balanced.

20 gallon tanks are generally very good as far as dimensions go, but some tanks are very tall and very slim. Without much depth, these tanks should be avoided as the dimensions don’t provide good swimming space for most fish.

Slim tanks can also be difficult to perform maintenance on.

As with all second hand tanks, you will also want to look for any cracks or areas where the silicone is very lacking or damaged. The silicone on a 20 gallon should be reasonably thick as it needs to hold a lot of weight.

If the corners look very bare, then the tank may need to be re-siliconed; you should test its water holding capacity before buying it.

What are the best 20 gallon fish tanks?

Every few years aquarium companies will release a new series of tanks to compete against each other. Oftentimes, they will be released in a variety of sizes, so every few years there are a bunch of new branded 20 gallon tanks.

Here we will list some of the most popular current 20 gallon aquariums on the market.

We will be listing ones we recommend, giving a non biassed review on each of them, including pros and cons of each tank.

Juwel Primo

The Juwel Primo series includes a couple different sized tanks, notably the Primo 110 and Primo 70 being 110 and 70 litres respectively.

The Primo 110 holds just over 20 imperial gallons and the Primo 70 is just under 20 American gallons.


  • Both of these tanks are excellent starter aquariums – they have good dimensions, allowing for a good amount of swimming space.
  • The tanks also usually come with a filter, heater and light.
  • The filter is a bioflow one filter, which is very good for this size of tank.
  • The filter also has a compartment which allows the heater to be placed inside it. This is great as it best circulates heat around the tank, is easily accessible and is the best use of space for the heater.
  • The tank itself is very sturdy and well built. As mentioned, the dimensions are good and the square shape allows for versatile placement.
  • The lights are an energy efficient LED strip and are quite good for growing plants, although not the best.


  • The tank has a weak plastic lid which can break under stress.
  • Juwel equipment is often shaped and made to fit specific sizes, meaning that the tank is not very modular. The filter equipment and heater need to be specifically Juwel branded in order to fit the sizes.

Fluval Roma 90

The Roma 90 holds 90 litres, which is 20 imperial gallons or 24 American gallons. The Roma comes with a Fluval filter and heater as well as a bluetooth 7.5 watt LED light strip.


  • LED lighting controllable by bluetooth, energy efficient and very good at growing live plants.
  • Lid flaps are flexible, resistant to snapping and easy to remove.
  • Lid has holes in the back which allow for pipes and wires to seamlessly pass through.
  • Comes with a U2 or U3 adjustable Fluval filter.
  • Comes with an adjustableFluval heater.
  • Glass dimensions are great and are very professionally constructed, being very sturdy tanks.


  • The Fluval heater that the tank comes with seems to have frequent reports of breaking and don’t last very long, from our experience.

Marina LED Aquarium Kit

This series of tanks come in a few sizes with the LED 20 holding 75 litres / 20 US gallons or 16 imperial gallons.


  • The tank is a well dimensioned square shape, with pretty solid glass structure.
  • Comes with a hang on back filter with replaceable filter floss and carbon media.
  • Comes with an energy efficient LED light.
  • Comes with a heater.


  • Plastic fragile lid that can break easily.
  • Filter doesn’t have bioactive sponge, but instead floss, which needs to be replaced once a month or so.

Aqueal Leddy 

The Aqueal Leddy is another series of tanks that come in different sizes. For 20 gallons, there is the Leddy 75 for US measurements and the 105 for imperial.


  • Tank is well built and sturdy.
  • Comes with light, filter and heater.
  • LED light is energy efficient and strong enough to grow plants.
  • The LED also has a blue setting.
  • Suitable for freshwater and marine.


  • Lid is plastic and fragile.
  • Filters are only small and have had reports to lose effectiveness over time.

How many fish can I have in a 20 Gallon aquarium?

This question has a highly variable answer as there are many factors that will influence it; experience, types of fish, temperature, amount of times fed, whether the tank is planted or not and level of filtration.

However, in short, we would probably say try to avoid squeezing in more than 15 2” fish, but again this is heavily variable. For instance, in a planted tank, with experience someone can breed guppies and have hundreds of guppies in a 20 gallon without issue.

On the flip side, someone can have 2 baby goldfish in a 20 gallon tank and suffer from ammonia problems.

Having live plants, strong filtration, good aeration and performing regular maintenance will allow you to keep more fish in any aquarium, but the upkeep will be high the more you push it and neglecting a tank with a very high level of stock can be catastrophic.

A tank with only a handful of fish will be much less demanding, it will need feeding less, produce less waste and require much less maintenance, it will also be much more stable.

What is the biggest fish size you can put in a 20 Gallon fish tank?

When we talk about the largest fish we would squeeze into a 20 gallon tank, bear in mind that these are recommendations which will give you the best chances of success, and that are fair to the animal.

These recommendations are also considering the full grown size of the animal. Yes, these fish should be able to live their whole lives in a 20 gallon tank, although more space is always better.

You certainly can house a baby Redtail Catfish in a 20 gallon, but it will eventually grow to be larger than the aquarium itself, so do your research and make sure that the fish you choose will still be able to live in your tank when they are fully grown.

The largest fish we would probably squeeze into a 20 gallon tropical tank, even when fully grown, is a small Bristlenose Pleco; anything larger, we would recommend upgrading to a larger tank.

As far as coldwater, one or two Pearlscale Goldfish can probably live in a tank of this size with little issue.

For marine, a pair of full grown Ocellaris Clownfish will live comfortably in a tank this size.

Best fish for a 20 gallon tank

As mentioned, a 20 gallon aquarium gives you a lot more room to experiment with, and it gives you a wider variety of fish to keep. Here we will make a list including some of the ideal fish to house in a 20 gallon tank.

This list will contain fish for a variety of tanks, coldwater, tropical freshwater and marine. All the fish listed here will be able to live in a 20 gallon permanently as an adult with little issue relating to tank size.


Pearlscale goldfish

The smallest variety of fancy goldfish, a pair could be housed in a 20 gallon. They enjoy tanks with lots of aeration but little water movement due to their poor swimming capabilities, so an airstone is definitely needed.

Using sponge filters is advised as opposed to an impeller run filter that produces a current.

Zebra or leopard danio

The smaller varieties of danio do really well in 20 gallons – the extra space provides them with more swimming room, which danios will always appreciate due to their highly active behaviour.

Danios need high oxygen levels so an airstone or high surface agitation is needed. They also enjoy currents to swim against, but will do just as well in a tank without one.

Danios are a highly social fish and need to live in groups, you could fit around 10 in a 20 gallon quite comfortably.

Borneo loach

This fish is often mislabelled a hillstream loach depending on your area, but its real name is the borneo loach. It appears like a tiny stingray, black in colour with white dots all over its body.

This fish again needs high oxygen and will really benefit from a current being present in the tank.

Borneo loaches can also be difficult to feed at times as they live on algae and biofilm, so they will only really do well in mature tanks with plenty of roughage for them to graze on. They will also eat algae wafers, but need a mature tank to do well in.

Hillstream loach

Pretty much the same as the borneo loach, except they look like a miniature crocodile. Again, they do great in mature tanks, but not as well in newer, cleaner aquariums.

These fish are a little more difficult to find than the bornean loach, but they have pretty much the same care requirements.

Although they are small, both of these loaches do better in larger tanks as it gives them much more to graze on, which is why we recommend them for a 20 gallon as opposed to a 10 gallon or smaller.

Florida flagfish

Although more of a temperate fish, these guys can live fine at room temperature –  they mainly eat algae and veggie based foods and are relatively easy to keep.

They can however, be rather territorial and are renowned for fin nipping.

Variatus platy

The temperate species of platyfish, these guys do best in 20 gallons or larger and will breed easily if kept in a mixed-sex group.

They enjoy a higher pH and hardness level, so your water may need buffering with an aquarium kH buffer.



More specifically bronze, paleatus and panda corydoras as these are the smaller varieties that will do well in a 20 gallon. These fish are excellent for beginners and veterans alike, they are always entertaining to keep.

They need to live in groups as they are a social catfish; we would probably recommend keeping a school of around 6-8 in a 20 gallon tank.

They need to be fed sinking foods, catfish pellets, bloodworms, tubifex and algae wafers are all great foods for cories.


Platies do great in 20 gallon tanks, as the extra space provides them with more territory and makes disputes less frequent. Although they can be kept in a 10 gallon tank, we recommend a 20 gallon as it removes threats of male squabbles and fin nipping.

Platies are very easy to keep and breed – they will eat pretty much anything, but need a diet with more vegetable and plant based content. Flakes, algae wafers, frozen food, boiled veg, pretty much anything is game.

If you do keep platies and you have a naturally low pH in your tap water, then we recommend buffering your water with a mineral. Uou can use a dedicated aquarium buffer or bicarbonate of soda for this, as it will add calcium, bringing up the hardness and pH.

Khuli loach

Kuhlies are an extremely fun fish to keep in a 20 gallon, they are one of the smaller varieties of loach, and appear as long snake like fish with yellow and black bands all down their body.

These fish are quite shy and need lots of cover to dwell in, they eat sinking foods, similar to the corydoras catfish diet.

They do best in planted, mature aquariums in groups of about 7.

German blue ram

Ram cichlids are a family of small dwarf cichlids from south america – they are a good cichlid to keep in a tropical community tank due to their relatively peaceful nature.

These fish do occupy territories which they will defend, a 20 gallon is ideal as it gives them enough space to find their own territory where they can live comfortably, while still allowing other fish to live alongside them.

In a smaller tank, rams can cause aggression issues as their territory is easily intruded by other fish.

Multifasciatus (shell dweller cichlids)

Multis and other shell dwellers are a great cichlid for smaller tanks. They do excellent in a 20 gallon tank, they like hard high pH waters and like to live with catfish and other cichlids.

They make great additions to those specialist community tanks or breeding setups.


Ocellaris clownfish

A pair of clownfish will do great in a 20 gallon marine tank. They are reef safe and do well in community reef tanks provided they have enough space to set up their own territory.

Much like the freshwater dwarf cichlids, clownfish will aggressively defend their territory and in small tanks will fin nip and attack other fish, so a 20 gallon tank where they can find their own space and get away from other fish is ideal.


Firefish are another great fish for those smaller setups. They like to stay in groups, so keep around 4 or 5 of the same species, provide plenty of rockwork for them to hide in and they will do well in a stable marine tank.

They are reef safe and do great in 20 gallon tanks, but one essential tip is to keep a secure lid on, as they can and will jump out of the tank.

Tailspot blenny

Another reef safe fish that lives amongst corals and rockwork, in a 20 gallon, it is best to keep just one blenny as they occupy large territories.

They feed on mainly algae, so do best in mature, dirty marine tanks with plenty of surface area to graze on.

Red hawkfish

Hawkfish are great for 20 gallon reef tanks. They are largely immobile, perching on rocks and waiting for food to pass by, so they don’t occupy the upper swimming spaces.

They are good for communities, although they will eat small fish and invertebrates, shrimp, baby clownfish and crabs are not safe from hawkfish, but larger or similar sized fish are safe.

Best filter for a 20 gallon fish tank

Fluval U2

The Fluval U2 filter is suited for a tank of 110 litres. It is a high capacity internal power filter, containing chambers with ceramic media, sponge, floss and carbon.

The U2 is ideal for a 20 gallon tank as it filters a bit over the literage the tank holds, we always recommend over doing filtration to ensure stable water parameters 

The U2 is also incredibly versatile, sturdy and can be adjusted in flow rate.

Interpet CF2

The CF2 internal filter is another great power filter; it sits just above the surface and cascades down, much like a hang on back, but still inside the tank.

This filter is good for oxygenation and is easy to acquire parts for, it is affordable, works well and doesnt take up too much space.

All Pond Solutions 600IF

This is another great filter for a 20 gallon tank; it does up to 110 litres and is highly versatile, much like the U2 as it can be flipped on its side, allowing it to be housed in shallow water.

The filters are very sturdy and produce a good amount of current, excellent for oxygenation and keeping debris clear.

Sponge filters

Sponge filters come in all shapes and sizes, but they are perhaps some of the best filters out there. They are very cheap, extremely versatile, easy to run and have maximum efficiency when it comes to filtration and oxygenation of an aquarium.

Sponge filters are very simple. Air is run through a tube surrounded by sponge, and as the air is forced through, it pulls water through the surrounding sponge as it displaces it going up through the tube. 

This then filters the water as it is drawn through colonies of beneficial bacteria. It also at the same time aerates the water as the air bubbles rise to the surface, disturbing the waterline and allowing oxygen and carbon dioxide to diffuse.

They don’t produce a current, meaning that they are ideal for fancy goldfish and bettas who are poor swimmers.

They are run with an air pump and depending on the amount of sponge filters you have can mean you need a larger or smaller air pump.

Tetra APS

For just one or two sponge filters in a 20 gallon, the APS 100 is ideal as just a stand alone air pump. It is quiet, energy efficient and can be connected easily to air stones and sponge filters using airline tubing.

For a 20 gallon tank, you wont need more than 2 8” tall sponge filters, so an APS 100 will do the job just fine.

Best heater for a 20 gallon fish tank

The best way to calculate what heater is best for your tank is to figure out how many litres your tank holds. When calculating, you need to be aware that there is a difference between american measurements, and those used by most other countries.

imperial gallons are the more commonly used way to measure water volume across the world and hold a little more value than an american gallon.

  • Imperial gallons are the equivalent of 4.5 litres
  • American gallons are the equivalent of 3.8 litres

So when calculating, just remember that an american gallon is smaller than an imperial. 20 american gallons is the equivalent to only 16 imperial gallons.

A 20 gallon tank in America will be smaller than a 20 gallon somewhere else in the world because of this, so a different wattage heater can be used.

When applying the correct wattage heater to an aquarium, you need to match the number of litres to the number of watts.

  • 20 us gallons is about 75 litres, so a 75 watt heater can be used.
  • 20 Imperial gallons is about 91 litres, so we would suggest using a 100 watt heater.

Interpet Heater

For a 20 gallon tank, we recommend the interpet 100 watt. It is a very sturdy glass heater, they are clear to read and easy to use, they do the job of warming the tank very well.

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