Best 20 Gallon Fish Tank: How to Pick Fish, Equipment, and Set it up (2019 Guide)

20 gallon fish tank planted

A 20 gallon fish tank is a smart choice, especially if you’re a beginner.

You get lots of wonderful stocking options, it’s not so large that it takes up a lot of space or have high running costs, and it’s got a large enough body of water to help maintain stable parameters.

Here’s what you need if you want a 20 gallon fish tank and how to set it up.

Step 1: What Do You Want to Stock?

20 gallon aquarium

Your first step in planning your tank is to decide what kind of fish you want to keep.

Different species of fish have different needs. You’ll base all your other decisions on what kind of fish you stock, like: shape of the tank, filter size, decor, substrate, plants, etc.

Pro Tip: If you’re interested in a certain species of fish, make sure to do a lot of research. That way, you know if it can live in a 20 gallon, what other fish can be added to the tank and what that species needs as far as food, decor, temperature and substrate.

20 Gallon Tank Stocking Ideas

Here’s a list of a few species that can work in a 20 gallon. Of course, there are way more out there. These are just a few of my favorites.

Ram cichlids (Mikrogeophagus ramirezi) – dwarf cichlids that need a mix of open swimming areas, dense plant clusters and small caves. They prefer sand.

Panda cories (Corydoras panda) – adorable dwarf catfish. They like open swimming areas and sand substrate.

Platies (Xiphophorus maculatus) – prefer densely planted areas mixed with open swimming areas.

Cardinal tetras (Paracheirodon axelrodi) – slightly bigger cousin of the neon tetra, they prefer lightly planted tanks with tall plants to dart into.

Rainbow kribensis (Pelvicachromis pulcher) – use a sand substrate and provide several caves with planted areas and lots of decor so they can stake out a territory.

Oto cats (Otoclinus vittatus) – excellent cleaner fish that needs lots of plants and decor that it can scour for algae.

Harlequin rasboras (Trigonostigma heteromorpha) – they mostly want open swimming areas with a few tall plants at the back of the tank.

Building a Community...Tank

A lot of aquarists want to build a community tank, an aquarium that showcases several different species living together.

Water Parameters – it’s best to put fish together that have similar needs, like temperature and hardness.

Behavior – this is the biggest factor to consider when you’re trying to figure out which fish can live together.

Aggression – adding aggressive fish to your tank can spell absolute disaster. Some fish are just plain meaner than others. Do your best to match aggression levels so no one is getting beat up.

Schooling behavior – look to see if your fish prefer to live in big groups or if they’re loners. Schooling fish should be kept in groups of 5 or more so they feel happy and secure.

Territory – some fish like to stake out a little territory and defend it against other fish. You’ve got to make sure that territorial fish are going to have enough room to be happy so they’re not bullying everyone else into a corner because they feel crowded.

Swimming Level – not all fish live at the same level of the tank. You’ll see different species listed as top, midwater or bottom dwellers. It’s a good idea to mix fish which prefer different swimming levels. Then they can spread out all over the tank instead of everyone trying to crowd into one area.

Step 2: Choosing The Best 20 Gallon Fish Tank & Equipment

fish tank equipment

There are two different 20 gallon tank shapes: high and long.

A 20 gallon high is better suited to mid-water fish so there’s more room in between the surface and substrate.

A 20 gallon long is best for fish that primarily inhabit the top swimming area or live along the bottom. The bigger footprint of a 20 gallon long makes these two areas bigger.

Last update on 2019-11-08 / Affiliate links / Images from Amazon Product Advertising API

Get a Kit or Start from Scratch?

You might be thinking about picking up a kit so you can buy your tank and equipment all at the same time.

Some of the better name brand kits are OK. But, in my opinion, a lot of kits have low quality equipment that come with them.

Personally, I buy my tank, and maybe the lid, together. But the rest of it, I pick out my equipment from scratch so I get higher quality stuff.

Last update on 2019-11-08 / Affiliate links / Images from Amazon Product Advertising API

Choosing Your Filter for Your 20 Gallon Tank

All right, now that we’ve gone over your options for tank shape, let’s go over the essential equipment you’ll need for your 20 gallon: filter, heater, lighting and maintenance gear.

I really don’t recommend that you skimp on your filter. Bargain basement filters don’t do a great job and fail much sooner than higher quality brands.

It’s not much of a deal if you have to buy 5 cheap ones compared to a single good one.

Types of Filters

Hang-on-the-back – this is the most common kind of filter out there. My biggest concern when it comes to these is finding one with the best biofilter capacity, like the Aquaclear 20 or the Tidal 35.

Internal – these filters go inside the tank. Some you fully submerge and others are meant to have the top sticking out of the water. Internal filters are easy to set up and maintain and are really quiet.

Sponge – these are one of my favorites. They go inside the tank and work by forcing water through a large sponge with a powerhead or air pump. They’re great mechanical and bio filters that are cheap and easy to set up and maintain.

Understanding GPH

You’ll see filters rated for different sizes of tanks. What this is really talking about is the number of gallons the water pump in the filter can go through in an hour, the gallons per hour, or GPH.

You want a filter that can pump at least 4 times the capacity of the tank each hour.

So, for a 20 gallon, you want a filter with a GPH of at least 80.

In my opinion, I’d rather err on the side of over filtering. I usually go with a GPH that will turnover my tank about 8-10 times an hour.

But this can depend on the stock you choose.

How Waste Levels Relate to Your Filter

Some fish produce more waste than others. And you might be surprised to find out which ones.

Fish like zebra danios and neon tetras really don’t produce that much waste.

But then fish like guppies or platies are hyperactive eating machines that poop nonstop.

The more waste the fish produce, the bigger the filter needs to be. When in doubt, go with a larger filter to make sure it can process all the waste produced.

Flow Preferences for Fish

Not all fish like the same amount of flow in the tank.

Species like zebra danios love high flow and will play in the current all day.

Others, like bettas and dwarf gouramis, prefer a gentle flow so they don’t have to fight it all the time.

Last update on 2019-11-08 / Affiliate links / Images from Amazon Product Advertising API

Choosing a Heater for Your 20 Gallon Tank

In my experience, there are two main factors when you’re trying to choose a heater for your tank: average room temperature and quality of the heater.

What’s the Air Temperature?

A big factor on the size of heater you need is the average air temperature in the room.

If you live in a warm area, so the heater needs to raise the temperature 9°F (5°C) or less, then it’s probably safe to go with a smaller heater, like a 100 watt.

But, if the temperature in the room can dip down lower than 65°F (18°C), a 100 watt heater might struggle to keep up.

For a situation like that, I’d recommend going with a 200 watt to make sure the heater has the power it needs to maintain a steady temperature.

You Get What You Pay For With Filters

I really do not recommend going with a cheap heater.

They can develop all kinds of problems, like shattering, refusing to come on after a few months, or the absolute worst, getting stuck on and cooking all your fish.

Yes, that can actually happen, and yes, it’s as horrible as it sounds.

So a cheap heater might be cheaper now. But when you have to replace it early, or have to replace the heater and all of the livestock in your tank, suddenly that discount heater isn’t such a bargain.

Do yourself a favor, invest in a higher quality heater.

Pro Tip: For more information on the pros and cons of different heaters, take a look at this article on heaters.

Heater Features

Really nice bonus features:

Temperature dial – makes it easier to set and tweak the temperature.

Shut off sensor – detects when the heater is out of the water and shuts the heater off.

Indicator light – lets you know when the heating element is on.
Shatterproof exterior – I’ve had a lot of heaters shatter over the years, so now I stick with shatterproof.

Last update on 2019-11-08 / Affiliate links / Images from Amazon Product Advertising API

Choosing Lighting for Your 20 Gallon Tank

There are a lot of choices when it comes to lighting your tank:

Compact fluorescent

In my experience, you spend a bit more on LEDs when you first buy them, but they save you money over time. A good LED light can last a really long time before it needs to be replaced.

They also draw less power, so they save you money on your electric bill over time.

Fish, and especially plants, have different lighting requirements; they prefer more or less intense light.

Since 20 gallon tanks are so small, LEDs should be more than sufficient to give your tank all the light it needs.

Sizing Lights for 20 Gallon Aquariums

A 20 gallon high and 20 gallon long have different outside dimensions.

The dimensions of a 20 gallon high are 24” x 12” x 16” (61 x 30 x 41 centimeters). A 20-22 inch (51-56 centimeter) light would be good.

A 20 gallon long is 30” x 12” x 12” (76 x 30 x 30 centimeters). A 24-28 inch (61-71 centimeters) light would be best for this size.

Last update on 2019-11-08 / Affiliate links / Images from Amazon Product Advertising API

Maintenance Equipment for a 20 Gallon Tank

You need at least some gear to help you maintain your new 20 gallon aquarium.

Gravel Vacuum

I can’t stress enough how much you need this.

A gravel vacuum lets you siphon out water and get fish waste and other gunk out of your substrate.

For a 20 gallon, you really only need a gravel vac with a 9 inch (22 centimeter) lift tube.

Last update on 2019-11-08 / Affiliate links / Images from Amazon Product Advertising API


Having a few buckets on hand it essential. You’ve got to have a way to carry water when you’re maintaining your tank.

For a 20 gallon, I’d recommend two or three buckets. 3-5 gallon (11-19 liters) buckets should do the trick.

I usually go to the hardware store and get the empty 5 gallon buckets meant for paint. They’re cheap and really tough.

Pro Tip: Make sure to mark your buckets so they don’t get used for household cleaning. The residue from soaps and cleaning products is deadly to fish, even when the bucket has been rinsed really well.


Sponges are really useful for cleaning algae off the inside glass of your aquarium.

You can get whatever generic sponges are carried at your local grocery or discount store.

Just make sure that they don’t have any kind of added soap or cleansers.


This is an absolute must have.

Dechlorinator treats water and eliminates chlorine or chloramine that’s been added to your tap water.

There are a lot of different brands out there, but I prefer Seachem Prime.

Last update on 2019-11-08 / Affiliate links / Images from Amazon Product Advertising API

Useful Miscellaneous Equipment You Don't 100% Need

The following are things you don’t really have to own when running a 20 gallon fish tank, however they can be useful and good to be aware of.


Yes, you can get away with skipping a net if you needed to.

But they come in really handy, so I recommend them.

You can use them to add fish, catch fish that need to be moved or even remove fish that have died.


Lids aren’t absolutely essential, but I like them for a couple of reasons.

Cut down evaporation
Keep out dust and bugs
Stops goofy fish/invertebrates from leaving the tank


It’s important to monitor the temperature in your tank. It’s the only way to know if you need to tweak your heater, or if it’s just plain not working right at all!

I really like the JW Pet Magnet Smart Temperature Aquarium Thermometer. It’s easy to install and read. I’ve got one in all of my tanks now.

Last update on 2019-11-08 / Affiliate links / Images from Amazon Product Advertising API

Air Pumps

Air pumps force air through small plastic airlines and through diffusers called airstones. The airstones are put under the water in the tank where they create bubbles.

The bubbles help circulate and oxygenate the water in your tank, making it easier for fish to breathe.

I use airstones in all of my tanks, usually at the opposite end from the filter. This helps keep the water moving and cuts down on dead spots in the tank.

Surface Skimmer

These devices have an intake that floats at the surface and lets water from the surface flow in. Then the water passes through a mechanical filter that grabs any film or floating gunk before the water gets pushed back out of the unit.

They can attach to your existing filter or be standalone units.

They’re helpful for keeping the surface clear, but they can inadvertently grab flake food off the surface before fish can eat it.

UV Sterilizer

A UV sterilizer passes water right up against a bright UV light. The UV kills bacteria, viruses and free-floating algaes.

Not only do they help keep your tank healthier, they can also help to keep your water looking nice and crystal clear.

Pro Tip: Just like with heaters, you get what you pay for with UV sterilizers. Cheap units for sale online really don’t do that much.

If you want to shop for UV sterilizers that have been tested in an aquarium maintenance business, take a look here.

Test Kit

You really need to test your water frequently the first few months your tank is set up.

You need to know what your water parameters are and what you need to address.

And I would urge you to go with liquid test kits instead of test strips. Liquid tests are more reliable and have a longer shelf life than the strips.

API makes a Freshwater Master Test Kit that has all the basic tests you need. I use one of these for my tanks.

Last update on 2019-11-08 / Affiliate links / Images from Amazon Product Advertising API

Step 3: How to Set Up Your 20 Gallon Tank

setting up fish tank

1. Place your aquarium on a flat, level surface. Make sure that the entire base of the aquarium is evenly supported, or it could develop leaks over time.

2. Rinse your substrate. No matter what kind you choose, all substrate ends up with dust in the bag that can make a huge mess in your tank.

To rinse it, I usually put about ⅓ of the bag of substrate in a 3-5 gallon (11-19 liter) bucket. Run water from a garden hose into the bucket and let it overflow. Gently stir the substrate with your hand and keep running the hose until the water comes out clear.

3. Drain most of the water from the bucket and gently place the substrate in the bottom of your tank. You usually want about a 1 inch (2.5 centimeter) layer across the entire bottom.

4. Place all of your equipment in the tank, like the heater, filter, airstone, etc. But leave it all unplugged for right now!

5. Add in all of your decor. Do your best to make sure that none of it touches your heater. For live plants, you might want to wait until you’ve got the tank about half full of water.

6. Start filling the tank with water. Pour the water very slowly so you’re not digging out the substrate. This can make an even bigger cloud of stuff in the water column from kicked up substrate dust.

Once you’ve got about half the tank filled up, add your live plants that go down in the substrate and finish filling the tank.

7. Add dechlorinator and start your equipment up. Check the directions for your filter to see if you need to prime it.

8. Place your lids and lights on the tank and get them adjusted.

9. Over the next few days, monitor the temperature and make any tweaks to the heater that are needed.

10. And now onto cycling!

Pro Tip: Don’t worry if the tank looks super cloudy and/or all the surfaces seem covered in tiny bubbles at first. It’s totally normal and should clear up in a day or so.

Cycling the 20 Gallon Tank

Getting a nitrogen cycle up and running in your tank is one of the most important steps.

In case you’re not familiar with this, let’s go over the nitrogen cycle basics.

When you add fish to an aquarium, they immediately start to put off waste.

It would be so much easier if you could just toilet train them, but alas, no.

All that poop and pee sinks to the bottom of the tank and starts to break down, which puts off ammonia (NH3).

This is bad news because ammonia is really toxic for fish and invertebrates. All it takes is an ammonia level of 1 part per million (ppm) to stress and kill many kinds of fish.

With nothing to stop the ammonia, it would just keep building up until the tank was filled with toxic soup that kills everything.

No bueno.

But there’s good news, too.

There are beneficial bacteria that will eventually colonize your filters and substrate and process ammonia for you.

One kind of beneficial bacteria, turns ammonia into nitrite (NO2 -1) and another kind eats nitrite and puts off nitrate(NO3-).

Nitrate is much less toxic than ammonia or nitrite. It can be allowed to build up a bit in between water changes without hurting your fish.

This whole process, waste going from ammonia to nitrite to nitrate is known as the nitrogen cycle.

So yay! You have little invisible buddies who will happily help you keep your aquarium water from becoming toxic waste.

But, there’s a catch (isn’t there always a catch?).

These nifty beneficial bacteria don’t just show up in your tank instantly. You have to provide them the right conditions and then they’ll eventually move in.

This is what us old pros refer to as “cycling” a tank.

There are no shortcuts when it comes to getting a nitrogen cycle going in your tank.

Pro Tip: Don’t be fooled by products that claim they can speed up cycling your tank. Most of them don’t work.

Performing a fishless cycle is the easiest and most humane way to get a nitrogen cycle going in your aquarium. For the full steps on how to do it, check here.

Adding Fish

Once you have your tank fully cycled, it’s time to add fish.


But, you’ve still got to be careful and patient (I know, I’m no fun).

Adding too many fish too quickly can overwhelm the beneficial bacteria in your tank, causing harmful ammonia spikes.

It’s better to only add a few fish at a time so the bacteria can adjust to the new bioload.

After a few weeks, the colonies of beneficial bacteria will have grown to accommodate the higher bioload, and it’s safe for you to add a few more fish.

It’s better to add the least aggressive fish first and the most aggressive fish last. That way, the less aggressive fish have already established themselves in the tank before any potential bullies show up.

How to Add Fish to a 20 Gallon Fish Tank




1. Float the unopened fish bag in the top of the tank for 15-20 minutes so the temperature can equalize.

2. Place the net over an empty bucket. If you can get someone to hold the net for you, it’s really helpful.

3. Remove the bag from the tank and cut open the top.

4. Carefully pour the bag through the net, fish and all.

5. Quickly, place the fish in the tank and discard the water from the bag.

Maintaining Your 20 Gallon Tank

No buts about it, all aquariums require maintenance.

The biggest thing is that old water has to be removed and new water put in to replace it. This removes nitrates that can eventually build up to toxic levels.

Plus, live plants need to be pruned, algae needs to be scrubbed off the glass and filters need to be cleaned out.

The more fish you have in the tank, the more frequently you’ll need to do water changes.

Generally, if your tank is reasonably stocked, you only need to do a single 25%-50% water change each week.

It’s a good idea to use your gravel vacuum and get as much of the gunk out of your substrate as you can every time.

Stability is the Most Important Thing

Don’t try to dump a bunch of chemicals into your tank so you can make the water match the “ideal” water parameters you see listed for your fish.

Most tank bred aquarium fish have been raised for generations in all sorts of different conditions. So they can acclimate to your local conditions and do just fine.

What they can’t handle is the water parameters ping-ponging all over the place as someone tries to make their water “perfect” with tons of chemicals.

Maintaining clean water with stable water parameters is a much better way to have happy and healthy fish.

Conclusions on Setting up the Best 20 Gallon Fish Tank

Phew, I know, that’s a lot of information.

Hopefully, you feel like you have a bit more direction on how to get an awesome 20 gallon tank set up.

Research your species so you know what kind of tank and equipment is going to work best.

Don’t skimp on equipment. Making a bigger investment now will pay off when you don’t have to constantly replace the stuff.

And enjoy this process. Setting up a new tank has almost infinite possibilities. You’re creating a living work of art for your fish to hang out in.

That’s pretty exciting!

Katherine Morgan

Hey, there! I'm Katherine from Northwest Florida. I've kept aquariums for over two decades, enjoy experimenting with low-tech planted setups and an avid South American cichlid enthusiast.
Katherine Morgan

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.