Setup Ideas For Your 20 Gallon Fish Tank


Stocking your 20 gallon fish tank with fish, plants, and decorations is where you make it your own personalized creation.
Whether it’s a freshwater or a saltwater aquarium, research will need to be done to avoid errors.

Last update on 2024-07-20 / Affiliate links / Images from Amazon Product Advertising API

This is the most important place in your new hobby where you definitely want to dodge making mistakes in order to avoid fish loss.

How many fish can you put in a 20 gallon aquarium?
What are the best fish species to put in a 20 gallon freshwater tank or saltwater tank?
What about fish compatibilities?
Can I put live plants in my 20 gallon aquarium?
How do I stock a 20 gallon reef aquarium?

So many questions, right? This is perfectly normal.
All this can easily become overwhelming because of the multitude of possibilities available to you. There are literally hundreds of ways you can fill your aquarium. Let’s take a look at some of these different ways so you can make the best decisions for your 20 gallon aquarium.

Minimum Equipment Required

20 gallon tanks often come in all-in-one kits which include everything you need to get you up and going.
We feel like this is a decent starter and beginner tank size.

Remember that your tank will need to be cycled before adding fish, plants, and any other living organism you are planning to add to your aquarium.

Here is a great and easy-to-read article that explains aquarium cycling. The guys at say if they renamed this article, they would have titled it “How to Not Kill Your Fish”.

Here are the key things that you will need in order to get your tank ready for your fish.

1. Filter

The filtration systems that are provided with these all-in-one kits are usually good enough to get you started, but sometimes need to be upgraded depending on the fish communities you choose to put into the tank.
If you choose bigger fish, they naturally produce larger amounts of waste that needs to be cleared from the water and therefore makes the filter work harder.
They usually come with hang-on back power 3-stage filters and these are the best for a 20 gallon tank.

2. Water Heater

Sometimes freshwater kits don’t include a water heater or thermometer, so make sure to purchase one if the fish that you choose to add possess a certain water temperature requirement.
Most freshwater fish live well in water temperature of about 78 degrees, so if you can’t provide this temperature naturally in your home or office, then a water heater and thermometer will be essential.

Last update on 2024-07-20 / Affiliate links / Images from Amazon Product Advertising API

3. Lighting

Lighting is crucial for your tank not only to make it enjoyable to look at, but also to keep fish healthy.
Try to have a light that will mimic natural sunlight and moonlight if you can.
LED lights are now the most popular lights in the aquarist world for their quality of light, efficient and cheap power consumption, and durability.
Some kits will offer LED lights that come with the sunlight/moonlight feature.
If you plan on adding live plants to your aquarium, then the choice of light is even more important as plants require phosphorescence for growth.
if you are considering a reef tank, corals will require different LED lighting.

4. Substrate (gravel, sand)

Filling the bottom of the tank with gravel or sand not only provides a more pleasing look to your tank but also ensures a place for waste to be stored and eventually filtered.

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5. Plants and Decorations

Plants are a personal choice. There are a variety of plants to choose from, and again, many factors can affect the choices that you will make.
Some fish like to eat certain plants, so obviously you need to know that before stocking.
Fish usually enjoy places to hide or play in, so adding this element should be taken seriously.
If you feel uncertain about adding live plants at first, there are artificial ones sold that can fill your space nicely and provide fish with areas to enjoy.

Now let’s add some fish!

Fish stoking Guide for 20 Gallon Tanks

Here are a few ideas of fish that can live well in 20 gallon tanks.
There are many different formulas and combinations that can work, obviously.
Many factors need to be taken into account when thinking about the fish that you want to put into your new tank.
You can’t just go to the fish store, pick the fish that you find pretty, throw them into the tank and expect that they will live harmoniously and also ensure that they will not outgrow their space.

A 20 gallon tank is an excellent starter aquarium for a beginner or if you have limited space. It is plenty big to explore aquarium keeping to its full extent and for you to enjoy a diverse and unique aquatic world of your own design.

Many things need to be taken into consideration; like how well certain species of fish live amicably together, how big the fish will get at maturity, how many you can put into a 20 gallon tank, fish species layering, and also the pH level and other water factor considerations.


There are literally so many different ways that you can stock your aquarium that we couldn’t possibly go through all of the combinations.
What we can do is offer ideas of the best fish for beginners and the types that live well in smaller spaces, like your 20 gallon tank.

The fish that is most recommend for beginners are the livebearers. These remain fairly small at maturity and are easier to take care of.
Here are a few examples of livebearers to consider for your 20 gallon tank:

  • Guppies

  • Platies

  • Swordtails

  • Mollies

Other fish that are considered good choices for beginners with small tanks:

  • Neon Tetras

  • Zebra Danio

  • Cory Catfish

  • Bristlenose catfish

  • Tiger Barbs

Here are a few tried and trusted combinations of stocking ideas to get you started from

  • Livebearer combo: 3 x 2″ Guppies + 2 x 2.8″ Platies + 2 x 4.8″ Mollies.
  • Tetras combo: 5 x 1.2″ Neon Tetras + 5 x 1.8″ Glowlight Tetras + 3 x 1.8″ Black Phantom Tetras + 1 baby Albino Bristlenose Catfish.
  • 10 x 1.2″ Neon Tetras + 6 x 1.25″ Corydoras + 1 Albino Bristlenose Catfish.
  • 5 x 2″ Zebra Danio, 5 x 2″ Harlequin Rasboras, 5 x 1.25″ Corydoras.
  • 5 x 4″ Tiger Barbs.
  • You can replace each other between Tetras – Glowlight Tetra (1.6-2″), Black Neon Tetra (1.6″), Rummynose Tetra (2″), Black Phantom Tetra (1.8″), Diamond Tetra (2″).
  • For Zebra Danio, you can choose between Zebra Danio, Glowlight Danio, Gold Danio, Long Finned danio, Leopard Danio, Brachydanio rerio var. frankei, Pearl Danio, Celestial Pearl Danios, Danio marginatus, Gold Ring Danio.

Saltwater/Reef Tank

Watch this short video talking about saltwater fish that are good for beginners and excellent for small aquariums.

Snails and Shrimp

Snails, shrimps, and other invertebrates can be a fun and useful addition to your 20 gallon aquarium. They offer entertainment as well as provide cleanup duties. They do a great job at keeping the tank clean by naturally feeding on fish waste, algae, and other debris.

Take a look at this useful video showing the different types of snails and which ones are a good choice for the smaller aquariums and which snails can cohabitate with the small fish species for your 20 gallon tank.

Fish compatibility

This is one of the most important things to consider when choosing your tank mates.
Introducing new fish that commonly don’t get along will cause unnecessary stress to your fish and possibly fish loss.
Some fish species are aggressive and tend to bully smaller fish. You need to know all of this when planning your stocking options.


You can use a chart like this one to compare fish compatibility for your freshwater tank. This chart is not specific to fish that are good for 20 gallon tanks only. It is an extensive chart with several species combined. Refer to the fish guide we provided above to select the fish that are good for the smaller aquariums.


Here is a chart with saltwater fish compatibilities. Use this chart for your saltwater or reef tank as a reference. Again, this is an extensive chart listing a lot of fish, including some that are not suitable for 20 gallon aquariums.

Fish Maturity

When selecting your fish at the store you must be aware of the approximate age of the fish and you must have already done your research to know the full size of the fish at maturity.
This is a very important factor to consider because you may end up with an overpopulated tank within a few months. Over population causes stress on the fish and as space becomes limited certain fish may become aggressive.

Quantity of Fish

A 20 gallon aquarium can stock quite a few fish, but preferably the smaller species. Certain species fair better if they are communal with a minimum amount of its same species. Some of the smaller freshwater species can live in communities of thousands in the wild, and need to be with several others to be happy and remain healthy. When selecting your stock, research the minimum amount of each species that you can introduce into your tank.

There is a common rule in the aquarium keeping world that some follow and some others don’t believe is accurate enough to trust. The water density rule of 1 inch of fish per gallon is one that we tend to use as a general rule. Of course, some variables are to be considered on a more global level. Check out this article to find out more.


This is also an important thing to consider as some fish live better at the top of the water, others prefer to hang out in the middle, and other species do well as bottom dwellers.
This is a significant aspect to think about as you don’t want to have all your fish at the bottom of the tank most of the time.
Layering your fish will also ensure that the total aquarium space is evenly occupied and distributed. This creates a visually pleasing aspect, but also will permit more fish to be introduced in a healthy manner.

PH Levels and Temperature

After your tank has been cycled and is ready to welcome your fish, you need to test the water to make sure the PH and nitrogen levels are appropriate for your selected stock. Each fish comes with a preferred living conditions chart. The fish need to have similar environmental needs.

Fish Genders

This aspect is important only for some species that tend to reproduce easily like livebearers. Some others will only reproduce in brackish water, so no need to worry about your freshwater tank becoming overpopulated. If you are not equipped or ready to have a spawning tank, then care needs to be taken when selecting the fish at the store.

Also, some males in certain species tend to be aggressive and territorial. Make sure that you don’t introduce two males that may end up fighting and creating a hostile environment in your aquarium.

Here’s a useful article describing how to determine the sex of fish.

How to Proceed with Stocking

Introducing your fish to your new tank is a procedure that should not be taken lightly. Coming home from the store and plopping your fish into your tank quickly will surely result in fish stress and possibly fish death. To avoid this unnecessary stress there are techniques to introduce your fish to their new environment.

You want to make sure that the fish acclimate before being introduced into the tank with other fish. There are several ways of doing this, but the easiest and most effective way is placing the bag with the fish inside into the water and gradually adding your aquarium water in the bag.
Here is an article explaining the drip acclimation technique extensively.


Plants are always a great addition to any tank, even your 20 gallon aquarium.
Live plants require little more research and will command some extra work and maintenance.
We feel that it is well worth the investment because an aquarium with live plants always looks more natural and also the plants provide a healthy addition to your water.
Fish love to hide and play in plants, so this is to be taken into consideration.

Watch this short video showing easy to grow beginner plants.


This is where you can get as creative and as crazy as you want! There are so many options of decorations and ornaments for aquariums on the market, it should be easy to find items that you enjoy.
Driftwood and rocks are always a good idea, and are the most common to place in aquariums. Some decorations and structures incorporate air pumps and have the capacity to provide air bubbles. This is not a necessity to preserve the quality of your water, but if you enjoy the look of the bubbles this can be a fun addition for you. Make sure to purchase all decorations and ornaments suitable for aquariums that won’t be toxic to your fish. It’s not a good idea to add objects to your water that are not aquarium approved.

Some like to recreate a scene or a landscape. Remember that the more decorations and structures you add to the water, the more you minimize space for your fish. Sometimes less is more in this department, especially that your tank is on the smallish side.

A Few Examples

Here are a few videos of successful and unique 20 gallon tank setups.
As you can see there are a multitude of possibilities and so many countless ways to personalize your aquarium.


Stocking your new aquarium with fish, plants, and decorations definitely is the most exciting part! Patience is needed in the beginning in order to make the environment livable and healthy for your fish to thrive. Take the time to cycle your tank and research your combination options.

There are a great many of ways to personalize your new 20 gallon aquarium. Just keep these helpful tips in mind when planning your fish communities.

Feel free to comment below. We would love to hear from you and hear your thoughts and suggestions on this topic. If you have a successful 20 gallon tank that is up and running, share your stocking plan with our other readers.

Have fun stocking your aquarium with your new aquatic friends!

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