Best Small Aquarium Fish: Top Picks for 3 to 20 Gallon Tanks

When considering what fish to keep in a small aquarium, it is important to choose species that will thrive in a limited space.

I will start with my recommendations for the smallest aquariums, which are typically around 3 gallons, though sizes can range from 2.5 to 3.5 gallons.

One of my own tanks, for instance, is a 3-gallon long.

Why Choose Neocaridina Shrimp?

My first choice is the neocaridina shrimp. I absolutely love several of their color variations.

Yellow is at the top of my list because it contrasts nicely against hardscaping.

Two very popular options are the blue dream and the red cherry shrimp, both of which are beautiful.

I prefer the red cherry and yellow varieties over the blue dream simply because I find them much easier to see.

Even in a tiny aquarium, they provide a lot of fun activity and are incredibly cute.

You can start with about half a dozen, and in no time, you will have many more.

I find creating a scape for them to be a lot of fun. I enjoy using small plants like Anubias nana ‘Petite’, various Bucephalandra, and micro sword, which is another fun, small-leafed plant.

Of course, any type of moss works well. For anyone curious about keeping neocaridina shrimp, I recommend looking into Flip Aquatics.

I support their business, and they offer a beautiful assortment of many different colors.

What About Heterandria formosa?

Another excellent option is the Heterandria formosa, also known as the Florida least killifish.

They are super tiny, making them a perfect choice. Few fish are suitable for tanks under 5 gallons.

I personally would not keep a betta or many other fish in an aquarium that small. The Florida least killifish, however, are practically microscopic.

As the seventh smallest freshwater fish in the world, they are a perfect and really cute choice.

Can You Keep a Mystery Snail in a Small Tank?

My third recommendation is a mystery snail.

I once kept some magenta mystery snails with my green neons, and I think it was a fabulous combination.

Any color of mystery snail can be fun, and you only need one for an aquarium of this size. Keeping a single snail creates a fun, peaceful, and very low-stress environment.

Which Three Fish Should You Consider for a 5-Gallon Tank?

When moving up to a 5-gallon aquarium, there are three fish I would recommend to almost anyone.

What Makes the Chili Rasbora a Favorite?

My recommendation would have to include the chili rasbora, one of my all-time favorites.

They are super small, and you could easily keep a dozen in a 5-gallon tank with plenty of room.

For such tiny bodies, they have a brilliant red color that offers so much visual impact.

Their color contrasts nicely with any kind of planted tank. They are personable, adorable, and just bright red.

How Does the Dwarf Rasbora Compare?

A fish that closely resembles the chili rasbora is the dwarf rasbora.

It is probably not as popular but is very similar.

They are equally as red, with a slightly different pattern, and are just a little bit larger.

Why Add Amano Shrimp?

Rounding out my choices for the 5-gallon aquarium is the amano shrimp.

While their larger size compared to neocaridina shrimp might take some getting used to, they are excellent at keeping an aquarium clean.

They are constant workers, and for that reason, they are always a winning choice.

What Are the Top Choices for a 10-Gallon Tank?

When it comes to the 10-gallon, I definitely have three absolute favorites.

Why Are Ember Tetras a Great Addition?

I will start with the ember tetra.

I do not think their color can be beaten.

They are a bright orange and a perfect size, allowing you to keep a nicely sized school in a 10-gallon aquarium.

The first thing anyone will notice in the tank is the ember tetras.

I especially love them because, like many aquarists who enjoy planted tanks, I find there is nothing like an orange or red fish to contrast with the greenery, making them visible all the time.

While some people might consider it a basic fish, many of my favorites could be described that way.

It is a popular fish for a good reason: its incredible color.

What Is a Good Bottom Dweller for a 10-Gallon?

I am going to add a fourth option specifically for the bottom of the tank.

I think corydoras are an excellent addition for any aquarium 10 gallons or larger.

The pygmy cory is the smallest species and is very interesting.

Unlike typical corydoras, they do not stay exclusively at the bottom; they will swim throughout the entire water column.

They are also known for creating goofy little piles of themselves on the substrate.

Because they are so tiny, you can get a nice-sized school even in a 10-gallon tank. A school of pygmy cories would be my go-to suggestion for a bottom dweller.

Why Are Celestial Pearl Danios So Popular?

If someone tells me they have a 10-gallon tank and wants a stocking suggestion, I will almost always recommend the celestial pearl danio first.

Also known as the galaxy rasbora, it is a beautiful little fish with a stunning pattern, making it resemble a miniature trout.

The pattern looks like a dark blue body covered in white stars.

The fish also has bright orange coloration along the top and bottom of its body, which is quite striking as it moves around the aquarium.

Should You Consider the Sparkling Gourami?

My third choice is the sparkling gourami. I love this fish, and in a 10-gallon tank, you could easily keep five or six of them.

I think their coloration is beautiful, and I enjoy watching their modified pelvic fins, which look like little arms, and their beautiful eyes.

They can be a little spunky towards one another, but they are stunning in blackwater setups and, of course, planted tanks.

I just do not think you can beat them.

Although they are sometimes called croaking gouramis, I have never actually heard any of mine make the noise, but that would be an added bonus.

What Are the Best Stocking Options for a 20-Gallon Tank?

For my final nano aquarium example, I will discuss the 20-gallon, which I consider to be the largest size in the nano category, although one could also include 22-gallon tanks.

Which Cory Catfish Pairs Well in a 20-Gallon?

To start, I have a recommendation for a bottom-dwelling fish that can be combined with any of my other choices for a 20-gallon tank.

I would absolutely pick a species of corydoras. One of my current favorites is the Corydoras adolfoi.

I think they are a fun little choice, and they remind me of panda cories.

They stay on the smaller side, and I absolutely love the little orange spot on their head.

Why Are Green Neon Tetras a Top Pick?

My top choice for a 20-gallon aquarium will not be a surprise to some.

I have to recommend the green neon tetra. It is just one of several wonderful neon varieties available.

Whether you are a beginner, returning to the hobby, or have been keeping fish for a long time, do not underestimate the classic neon.

I particularly love the green neons, as I consider them to have three different colors—green, blue, and purple—all in the same fish.

My combination of green neons with magenta mystery snails will always be a favorite.

There are also other neons to consider.

The standard neon is a fan favorite with its bright red and blue coloration that creates a beautiful effect moving through the aquarium.

The black neon is another great option; while more understated, it is extremely hardy, gets larger than some other neons, and has a subtle blue color.

Finally, the diamond head neon is another one to remember.

What Makes the Bolivian Ram a Stunning Centerpiece Fish?

My second recommendation is the Bolivian ram.

While there are many varieties of rams, such as the German blue ram, I have found Bolivian rams to be very hardy and have had the most success with them.

I find them to be my favorite because they look like miniature Geophagus cichlids.

I think most things are cuter in miniature, and the Bolivian ram is stunning for its size.

You only need a pair in a 20-gallon tank to have a centerpiece fish that is always active.

Their fins are always displayed as they show off their colors.

While they can be kept with other fish, I would recommend getting a pair of rams to be the main attraction.

Why Choose the Rummy Nose Tetra?

For my final choice, I could not make recommendations for a 20-gallon aquarium without suggesting the rummy nose tetra.

This fish gives you both red and blue coloration.

It is such a beautiful species that it proves freshwater aquariums can be just as colorful and stunning as their saltwater counterparts.

I have shared my top picks for nano aquarium sizes that I would recommend to almost anyone.

I am sure there are other great options out there, and perhaps some of my omissions are surprising.

Conclusion

For a 3-gallon aquarium, I recommend keeping small, active species that thrive in limited space.

Neocaridina shrimp are an excellent choice, with vibrant color varieties such as yellow, red cherry, and blue dream.

I find the yellow and red cherry types easier to spot and more engaging to watch.

The Florida least killifish, known scientifically as Heterandria formosa, is another great option due to its extremely small size and peaceful nature.

A single mystery snail can also be a fun and low-maintenance addition, adding personality without overwhelming the space.

In a 5-gallon tank, I suggest keeping around 12 Chili rasboras, which are lively and brightly colored.

Dwarf rasboras offer a similar size and charm with slightly different markings.

Pairing them with hardworking Amano shrimp helps maintain a clean environment while adding variety to the tank’s activity.

For a 10-gallon setup, I recommend a school of Ember tetras, which are small and colorful, making them ideal for planted aquariums.

Pygmy Corydoras are a perfect bottom-dwelling companion, often seen exploring in groups across the substrate and swimming throughout the tank.

Celestial Pearl Danios bring a unique pattern and coloration, while a small group of Sparkling Gouramis adds interesting behavior and contrast to the setup.

In a 20-gallon aquarium, I find Corydoras adolfoi to be an excellent bottom-dwelling species with attractive markings and peaceful behavior.

A group of Green Neon tetras stands out due to their mix of green, blue, and purple hues, creating a captivating display in motion.

A pair of Bolivian Rams can serve as an active centerpiece, displaying strong personality and vibrant fins.

Rummy Nose tetras complete the setup with their bold red markings and synchronized schooling behavior, making them a dynamic and rewarding addition.