How to Start A New, Simple 20-Gallon Fish Tank Setup

By Eddie Waithaka @aquariawise

Starting a new 20-gallon fish tank can be an exciting and rewarding experience. In this post are some basic steps and hacks to follow to get started

If you’ve always wanted a fish tank and are only getting to it now, a 20-gallon is a perfect size to start with, but you will need to do some research to help you understand what you need and how to bring everything together.

I often caution my readers not to take everything the people selling you the fish or tank recommend, because they don’t always give the best advice.


To start your 20-gallon tank, you must ask yourself these five questions!

  1. What are some ideal freshwater starter fish for a 20-gallon tank? You do not want fish that are too expensive or high maintenance.
  2. How many fish are good to start with in a 20-gallon?
  3. What is needed in the tank: Heater, filter pump, lights, stand, lid, sand or gravel, rocks, plants, wood, decorations, etc.
  4. Any other maintenance products and items you need: conditioner, medication, thermometer, light timer, buckets, hose, vacuum, test kits, water, bottled nitrifying bacteria, etc
  5. Do you know the nitrogen cycle and how to cycle a 20-gallon aquarium?

Once you’ve checked all the boxes in the list above, you can start your fish tank!

How to Setup Your 20-Gallon Fish Tank

Choose the right location: Select a sturdy, level surface that can support the weight of your tank when filled with water. The best option is to place your 20-gallon on a custom fish stand, although any flat and firm surface, such as a TV stand, will hold.

Avoid placing the tank in direct sunlight or near a window or a heat source because too much sunlight will cause excess algae growth, and the heat will warm your water more than your fish like.

Gather necessary equipment: You will need a 20-gallon tank, a stand, a filter, a heater, a thermometer, a substrate (gravel or sand), decorations, and appropriate lighting. You may also need a water testing kit and water conditioner.

Rinse the tank, substrate, and decorations thoroughly before adding them to the tank. Add the substrate-gravel or sand-and decorations, and then fill the tank with water. Install the filter and heater according to the manufacturer’s instructions.

Cycle the tank to establish beneficial bacteria in the filter (and substrate) to break down harmful fish and plant waste. You can cycle your tank by adding a source of ammonia, such as fish food pure ammonia, or hardy species for fish in cycling and testing the water regularly for ammonia, nitrite, and nitrate levels 20ppm. Once the ammonia and nitrite levels have spiked and fallen between zero and 0.25, the tank is cycled and ready for fish.

Planting your 20-gallon: If you want plants in your fish tank, add them before you fill your tank and add fish. Give your plants several weeks to establish. You may need to dry-start mosses, like Monte Carlo and Java moss, but other species will start submerged once they are installed in the substrate, floating, or attached to driftwood.

Add fish: Choose fish suitable for a 20-gallon tank and compatible with each other. Some popular options include tetras, guppies, mollies, and zebra. Avoid overstocking the tank and provide hiding places and swimming space.

Maintain the tank: Perform regular water changes (about 20-25% every 1-2 weeks), test the water regularly, and clean your subtrate and filter (and change) media as needed. Keep the tank clean and well-maintained to ensure the health and well-being of your fish.

What Would Be Good Freshwater Starter Fish for A 20 Gallon

Livebearers are good starter fish if you only want a few fish in your 20-gallon. Most are colorful, hardy and easy to maintain. They also are an excellent choice if you prefer fish you can breed, especially guppies.

Start with 4 to 8 livebearers in your 20-gallon tank and plenty of plants for cover. Use rocks and wood to add interest to your aquarium and to provide exciting spaces for your fish to explore.

If you decide to start with livebearers, consider either having…

Tiny schooling fish also make good candidates for a 20-gallon starter tank. They are easy to maintain, feed, and clean after, and most are hardy, apart from a few, like tetras, that are sensitive to water quality.

You can have up to 10 of these tiny fish in your aquarium. Only be careful not to overstock them and leave a lot of open spaces to allow them to swim in their spectacular group formations.

If you prefer having tiny schooling fish to livebearers in your starter aquarium, think of…

You can also add bottom dwellers singly in a planted 20-gallon or with your livebearer or a group of schooling fish. Ideal bottom fish perfect for newbies include:

I recommend 4 panda or Julii corys in a 20-gallon. You can do 6 to 8 pygmy corys if you keep up with water quality and depending on how many more fish you have.

You can probably fit 2 bristlenose plecos in a 20-gallon without much issue, but one is best if you have other small fish in your aquarium. Bushynose plecos are shy, so ensure you have plants and hiding spots in the tank.

NOTE that if you want showy active fish to dazzle your fish tank, bristlenose plecos are perhaps not the best fish to have. They are more gem fish that often remain hidden at the bottom of the tank.

Otocinclus are tinnier than bristlenose and cories, so you can have up to 10 in a 20-gallon, depending on how many other fish you have.

Otos are docile, easy to maintain, and will feed on algae in the tank. However, remember to target-feed them because sometimes algae run out, plus your catfish needs nourishment from other food sources.

Throw algae wafers or veggies, like blanched zucchini, in the tank to supplement your otos’ algae intake.

These finnies do not overfeed (like some tropical fish), so you do not need to worry whether the food is in excess.

Sometimes all you’ll want is a single pet fish in your 20-gallon. Perhaps you want it for your kid, or you only need a companion that is not too much in your space, like a dog, cat, or bird.

Well, you can have a…

Betta are small, robust, colorful, and showy fish with exciting personalities, flowy fins, and easy enough for beginners to care for. They are perfect for a 20-gallon tank with dense plants, rockwork, and driftwood.

Betta males are territorial and can only be kept alone or with a small group of fast-swimming nano fish and aquarium critters, like shrimp and snails.

Female bettas are not as aggressive and can live in pairs with a single male, but their colors are not as vivid, so most people prefer having a single male.

Add a little liveliness to your 20-gallon with a colorful cleaning shrimp and snail crew. These critters make perfect tankmates for non-aggressive nano fish and are easy to care even for young, first-time aquarium owners.

Another fish that can be kept in 20 gallons alone or with a small group of nano schooling fish is…

Pea puffers are fun little guys to have in a tank. They are entertaining and soothing as they dart around plants and wood, looking for a meaty meal.

Interestingly, there are not many aquarium owners who consider these fish for their 10 to 20 gallons tanks, so if you are one to go against the grain and stand out, get yourself a pea puffer.

Pea puffers might be aggressive, but many are also docile and will live with other fish in a community if there is enough food and water space for all your finnies.

You only need to ensure you do not add your pea puffer with snails or shrimp because they will turn your critters into a quick meal.

Freshwater aquarium shrimp and snails also reproduce rapidly, so you will always have several in your fish tank, even if your fish picks on some of them.

Even so…

To ensure most of your population survive, add plenty of plants to your aquarium and ensure your fish are adequately fed. You may also want large shrimp and snails that are impossible for fish to eat.

Lastly, note that snails can readily overpopulate your fish tank, so keep them in check. Add assassin snails to reduce the population of other species if they get out of hand.

Assassin snails are particularly fond of eating Malaysian trumpet snails.

Nerite snails do not reproduce in freshwater, so you can add them to your tank if you think other species might overpopulate your aquarium.

If you would love shrimp and snails in your 20-gallon, consider…

— Amano shrimp

— Ghost shrimp

— Red cherry shrimp (are pretty small and are often killed by aggressive tankmates)

— Bamboo shrimp (are filter feeders and need enough water and a source of waterborne particles to feed)

— Blue velvet

— Nerite snails

— Assassin snail

— Mystery snail

— Ramshorn snail

How Many Fish are Good to Start with in A 20-Gallon Tank

The number of fish you add to your 20-gallon depends entirely on the fish you choose from the list above. The old rule of thumb of a gallon of water for every inch of fish should be a good guide, but remember to leave enough space for your plants, rocks, wood, substrate, heater, and internal filters.

The fish bioload should be a factor too.

The more waste a fish can produce, the more water space you need, and the fewer you can keep in a 20-gallon tank.

A tank and filter can only handle too much waste.

Too much fish waste risks clogging your filter, and dirtying your water, resulting in ammonia and nitrite build-up, which is toxic.

Fortunately, most fish suited for a 20-gallon (see list above) have a light bioload, so that should not be a hurdle.

That said…

Consider the little space you leave between the water level and the top of the fish tank to keep the water from flowing over and your fish from jumping out of the tank.


When you start your 20-gallon tank, about 20 percent of your tank capacity will be reserved for equipment, rocks, wood, plants, fish mass, and substrate.

Another 2 to 5 percent will be the space above the fish tank, so you will only have about 70 percent swimming area for your fish, which is about 15 gallons.

In 15 gallons out of 20, you can keep…

See table below…

Fish TypeTank SizeNo of Fish
Mollies20 gallon3to 4
Guppies20 gallon6 to 8
Platies20 gallon3 to 4
Swordtails20 gallon3 to 4
Neon tetras20 gallon6 to 8
Cardinal tetras20 gallon6
Harlequin rasboras20 gallon6 to 8
White cloud minnows20 gallon6 to 8
Zebra danios20 gallon6
Celestial pearl danios20 gallon6 to 8
Chilli rasbora20 gallon6 to 8
Ember tetras20 gallon6 to 8
Beckford pencilfish20 gallon6 to 8
Otocinclus catfish20 gallon6 to 8
Cory catfish20 gallon3 to 4
Bristlenose pleco20 gallon1 or 2
Kuhli loach20 gallon4 to 6
Betta20 gallon1 male, 2 females
Pea puffer20 gallons1 or 2
Snails20 gallonup to 10
Shrimp20 gallonup to 10

What Do You Need for A 20 Gallon Fish Tank

To set up a new 20-gallon fish tank, purchase the tank kit and all accessories necessary to keep your fish alive. These items include filters, substrate, heater, light, etc.

See the whole list below…

What Kind of Filter Do You Need for A 20 Gallon Fish Tank

A filter is one of the items you cannot overlook while setting up any aquarium, including your 20-gallon starter tank. An internal filter like the Top Fin Silent Stream or an Aquaclear power filter will work, but if you need a unit with more space for media, you can go with a Fluval or Eheim canister filter.

Sun sun filters can also be an ideal choice.

A larger, more powerful filter is better. It will provide more surface area for bacteria to live, and best when you have an overstocked tank. It will also improve water quality and help your fish and plants thrive.

Any of these filters (discussed below) will make a perfect choice for your starter 20-gallon tank.

Top Fin Silentstream Power Filter

The top fin silent stream is a small power filter suitable for 20-gallon tanks. You will want to get the one rated for a 30 or 40-gallon for best filtration.

The beauty of this filter is that it is available in most chain and pet stores, including Walmart and Petco, so you won’t have to hustle too much to get it.

Installing this filter is easy, and it comes with a decent hose that will easily reach inside your fish tank and a HOB unit easily mountable on the back of a 20-gallon tank.

The impeller is also easily accessible at the aft end of the hose, where it slots into the HOB unit. The filter media goes into the unit from the top part, which is equally uncomplicated to place and position, even for a newbie.

This filter features a four-stage filtration and can filter up to 200 gallons of water per hour.

At purchase, you will get a power filter unit with a bio and small cartridge and an adjustable intake tube (hose), which you can place at your desired height inside the tank.

You will get your top fin silent stream in black, and most parts being plastic.

Perhaps the only downside with this (and others) HOB filter is they are prone to flooding if they clog up, plus the waterfall effect can cause noise and splashes if your water is too high or too low.

HOB filters are not a favorite in the hobby, with most aquarists preferring to use sponge filters in smaller tanks and canister units in large aquariums.

Fluval Aquaclear Power Filter 50

The Aquaclear 30 power filter is another unit ideal for a 20-gallon fish tank, with a maximum output of 200 gallons per hour.

The filter is generally one of the most solid filters on the market and is reliable and recommended for tanks of up to 50 gallons. It has an adjustable flow, so you can reduce or increase it according to your needs.

An Aquaclear power 50 has multiple levels of filtration with almost perfect biological and mechanical media. Often, you’ll get step 1 and step 2 media included in the box.

Another pro of these aquarium filters is they are relatively quiet than others in their class but are primarily more costly.


Aquaclear filters are known to stop the impeller and cease working if left to run dry, so ensure you always have water inside the unit. If it stops running, use a small object, like a toothpick, to restart the impeller.

A sand substrate can also cause your Aquaclear filter to grind down, so check it for grains and clean it periodically.

Your filter will come in several plastic parts, such as the housing that holds the water, media, and motor, some of which might be brittle.

The filter media and other removable parts are easily accessible, so cleaning and replacing parts should not be too challenging.

Fluval 107 Canister Filter

A Fluval 107 canister filter is another good option for your 20-gallon tank. It is easy to use, even by beginners, and is generally well-regarded by customers for its quiet operation and effective filtration.

It is the best choice if you do not want a sponge or HOB filter for your 20-gallon, as may be the case when you have fish with a high bioload or if your aquarium is overstocked.

The Fluval 107 canister filter has a maximum flow rate of 145 gallons per hour, which is more than enough for a 20-gallon tank, which is adjustable to suit the need of the fish and plants in your fish tank.

Perhaps the only downside of this filter is it does not have as much space for the filtration media as larger canister filters, plus many tank owners have a challenge getting it to start when new or after cleaning.

Usually, the problem is caused by air in the hoses and filter reservoir, so remember to remove all the air in the system before you prime it. Before you assemble it, put it in a sink and fill with new water, bang and tilt it gently to get the air out, and put the lid on, displacing extra water into the sink.

Then fill the intake stub with water, hook it up, then turn it on while tilting the filter to get it started.

As you would expect…

The Fluval 107 canister filter has three filtration stages.

The first stage is mechanical filtration, which includes a foam pad that captures large particles and debris. The second is the chemical filtration stage, which uses activated carbon to remove impurities, odors, and discoloration.

The third stage is the biological filtration stage, which includes Bio-Foam that provides a large surface area for beneficial bacteria to grow and break down harmful ammonia and nitrite in the water.

Hygger Sponge Filter

Hygger double sponge , and similar filters are a perfect option for a 20-gallon tank. They are inexpensive, easy to set up, and effective at maintaining water quality.

These filters are especially perfect for a breeder 20-gallon or a tank with fish like betta and critters, like shrimp, that prefer a low water current. They provide a gentle flow and good biological filtration.

The only downside of using sponge filters is they must be purchased and used with a separate air pump, like the tetra whisper.

Best Light for Your 20-Gallon Fish Tank Setup

Chihiros WRGB Vivid 30 or A-Series

Overall, the Chihiros WRGB Vivid and A-Series are high-quality LED aquarium light fixtures with excellent performance for planted aquariums. Their full-color spectrum, energy efficiency, and remote control make them a top choice among aquarists.

These lights will work best in a planted 20-gallon, especially if you plan on plating your fish tank with demanding plants, like red species and heavy feeders.

Use Chihiros lights when setting up a high-tech 20-gallon planted tank with added fertilizer and CO2 for your plants.

Here is a review of its features and performance:


— The Chihiros WRGB Vivid and A-Series have a full spectrum of colors, including red, green, blue, and white LEDs, which can be adjusted to different intensities.

— They come with a remote control for adjusting the light’s temperature, brightness, and mode (sunrise, sunset, etc.).

— The fixture is slim and sleek, with a low profile that won’t detract from the aquarium’s aesthetics.

— The light is designed to provide high PAR (photosynthetically active radiation) output for growing healthy plants in a planted aquarium.

— The Chihiros WRGB Vivid is quality, energy-efficient, and has a long lifespan. Performance:

— The Chihiros WRGB Vivid and the A-Series lights provide excellent color rendition, making aquarium plants and fish look vibrant and natural.

— The light is strong enough to penetrate deep into the aquarium, providing sufficient light for all levels of plants.

— The fixture produces minimal heat necessary for maintaining a stable water temperature in the aquarium. The remote control is easy to use and allows precise light output adjustments.


— The Chihiros WRGB Vivid and the A-Series lights provide excellent color rendition, making aquarium plants and fish look vibrant and natural.

— The light is strong enough to penetrate deep into the aquarium, providing sufficient light for all levels of plants.

— The fixture produces minimal heat necessary for maintaining a stable water temperature in the aquarium.

— The remote control is easy to use and allows precise light output adjustments.

Fluval 3.0 plant LED Planted Aquarium Light

This light features a programmable 24-hour light setting and optional habitat light configuration for your tropical fish and aquarium plants best for a 20-gallon setup, for starters and budget hobbyist.

The light is best when you need plants in your 20-gallon setup but only want to start with simple and easy low to moderate light plants, like Java moss.

The ligth is also best when you have skittish loaches, catfish, and shrimp because the intensity is perfect for plants, but no too much for your fish.

You can use the FLuval app to manipulate the embedded device controller, which is very flexible and easy to preset for your Monte Carlo plants environment.

The initial cost of this light is high but justifiable because you won’t need additional hardware and software, plus it’s a higher-build than most planted tank lights.

The Fluval Aquasky Light will also do in a 20 to 55-gallon (or smaller beginner) planted tank, though it is not a plant-specific version.

Finnex Stringray

The Finnex Stringray is a perfect light for your 20-gallon freshwater planted aquariums. It provides bright and natural-looking light, decent for planted tanks.

This light may not be the best for high-light plants, but it is affordable and best for beginners and budget-oriented hobbyists. It is not as sophisticated as the Chichiros or Twinstar LED light but will work almost as good as a Fluval 3.0.

The Finnex stringray compares well to lights like:

— Nicrew

— Beamworks

— Hygger Full Spectrum Planted Aquarium Light

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