Fantastic Freshwater Nano Fish

Author: Mark Denaro

The most significant trend in the fishkeeping hobby continues to be the move toward nano tanks. While small aquariums come complete with their own set of issues, such as temperature control and more maintenance when you look at time spent per gallon, their challenges to the aquascaper in all of us cannot be denied. If you’re setting up a nano freshwater aquarium, chances are that it will include live plants. The style can be Nature Aquarium, Leiden, or biotope, or it can be somewhere in between, but one of the most interesting questions is what species to house in the tank.

Forget the plants and invertebrates for a minute and consider all those possible fish. It’s easy to rule out the ones that grow longer than the tank you’re using, but choosing among the smaller species can still be difficult. To help you make that decision, let’s look at ten of my favorite fish for nano aquariums.

Endler’s Livebearer (Poecilia wingei)

Endler’s livebearer (Poecilia wingei), also jokingly referred to as the endless livebearer, is related to the guppy but is a distinct, smaller species. The original wild-color form of this species is quite beautiful. Well, the males are anyway. As with guppies, they are much more colorful than the females.


In fact, this fish’s close relationship with the guppy is the biggest drawback to the continuation of its species. It may be extinct in the wild, and the frequent hybridization with the guppy in captivity to create new color patterns may make it impossible at some point to find true, wild-type fish. While all these newfangled color patterns are interesting, I would encourage you to seek out fish with the original wild-color pattern.


The males are absolutely spectacular with their green, orange, and black markings. Plus, you get the big upside of helping to maintain the original strain.


Females can grow to 1¾ inches (4.5 cm); the males stay significantly smaller. Females should outnumber males so that some can rest while the males are flirting with the others. There are some reports of females being a bit territorial toward each other, but I have not seen that. However, there is a dominance hierarchy, and that may be the behavior to which they are referring.


Endler’s livebearers are easy to breed and tend to get along well with other similarly sized fishes. They prefer their water to be somewhat hard and alkaline, and are tolerant of salt. This makes them a poor choice for a blackwater setup, but if your water is on the high side of neutral, they should do well.

Scarlet Badis (Dario dario)

The genus Dario includes a number of species that make great choices for nano aquariums, with new species frequently reaching the hobby as collectors work new areas in places like Myanmar and India.

Color Patterns

The most colorful of the group is probably the scarlet badis (D. dario). Oddly, females seem to have two distinct color patterns. Some feature the same vertically banded red-and-blue pattern sported by the males. Others are drab with very little patterning. This makes sexing them more difficult than it might seem at first glance, but when a group is maintained, they seem to sort things out on their own.


While they can certainly be converted to prepared foods over time, D. dario should be offered small live foods while they’re acclimating to life in the aquarium. Newly hatched brine shrimp nauplii are almost always accepted. Other good choices are vinegar eels, microworms, size-sorted Daphnia and Cyclops, and most anything else that will fit in their mouths. Over time, they can be converted to frozen foods, and some specimens will learn to accept crushed flakes or tiny pellets.

Other than the small challenge of getting them to feed, maintaining them is pretty straightforward. There will be a definite dominance hierarchy, and males will establish territories, so their behavior can be pretty interesting.

Norman’s Lampeye Killifish (Poropanchax normani)

There are so many killifish that can be kept in nano aquariums that it’s way beyond the scope of this article to include them all. Most killies are kept in pairs, trios, or small breeding groups, and that’s not really what I’m after here, so my choice is going to be Poropanchax normani, commonly known as Norman’s lampeye killifish. This schooling species does best when kept in a group in a planted aquarium with tankmates that won’t outcompete them at feeding time. They also look their best in such a setup, helping to get the most out of what may seem to be subtle coloration.

Unfortunately, these fish seldom photograph well, but they show very well in a planted aquarium with their light-colored bodies, which vary from white to silver to tan, and electric-blue markings around the top of their eyes.

Salt-and-Pepper Cory (Corydoras habrosus)

What selection of nano fish would be complete without a bottom dweller? The salt-and-pepper cory (Corydoras habrosus) fills this niche nicely. As the smallest of the bottom-oriented Corydoras species, it is one of the best for 2 nano aquariums. Growing to a maximum of 1½ inches (4 cm), with few ever reaching such a size, this peaceful dwarf cory makes a great addition to a community.


When maintaining C. habrosus, take care in choosing a substrate. If the substrate has sharp edges, these entertaining little guys may cut their barbels, which results in significantly less cuteness. In addition, there should be some open areas of substrate where they can congregate. These fish are sociable and will do best when maintained in a group.

Additional Characteristics

Sinking pellets or other foods intended for catfish should be added to the feeding regimen to ensure that they receive a good, balanced diet. Two other members of the genus should also be considered, C. pygmaeus and C. hastatus. These are midwater schooling species that will spend some time on the substrate but most of their time free swimming. In nature, C. hastatus forms aggregations with similarly colored tetras, such as Serrapinnus kriegi and Aphyocharax nattereri. Whenever possible, it should be kept with at least one of these species.

Galaxy Danio (Celestichthys margaritatus)

The galaxy or fireworks danio (Celestichthys margaritatus) has just about everything you could want from a fish. This species is breathtakingly beautiful, easy to breed, and simple to maintain. It is, however, a true nano fish, with the largest females not reaching an inch (2.5 cm) and the more colorful and slenderer males remaining even smaller.

Fishy Naming

Taxonomically, this species is rather a mess, as the original description was rushed in part so the author could impose the common name “celestial pearl danio.” Well, scientists don’t get to choose common names; collectors, exporters, importers, and hobbyists get to do that. Besides, we already have a pearl danio, and it’s a very nice fish in its own right. Subsequent scientific study invalidated the genus Celestichthys and moved the fish into the genus Danio. Later work revalidated Celestichthys and returned the fish to that genus, where it currently resides.

Physical Characteristics

The color pattern is truly reminiscent of a nighttime sky or even fireworks. I personally prefer the name “fireworks rasbora” because of the squiggly lines that appear on the body and in the fins. Whatever you call it, this species is ideally suited to a nano aquarium. It is an egg scatterer and will spawn in the aquarium, particularly among fineleaved plants. It does like to hide among floating vegetation but does not require that to thrive.

Fry and Socialization

It is possible that a few fry will grow up in the tank with the adults, but the chances for that seem to be higher in larger tanks with more hiding places. This species shoals rather than schools and some intra-species aggression will be seen. Males will fight with each other and display to females. They may occasionally nip each other’s fins.

Green Neon Rasbora (Microdevario kubotai)

The green neon or yellow neon rasbora (Microdevario kubotai) is another sparkling gem from Southeast Asia’s rainforest rivers, where it occurs in small schools. Whether the color appears green or yellow depends on how the light is reflecting off the fish and, to some extent, how you see color.

Physical Characteristics

Whatever color it appears, this is a beautiful fish in a tiny package. Females can reach about ¾ inch (2 cm), while the slenderer and more colorful males will stay smaller. Males will display to each other, as well as to females, so they’re also interesting behaviorally. Keeping a group of these is sure to make them one of your favorite fish.

Scorpion Betta (Betta brownorum)

Now, you had to know that we weren’t going to get through this list without any anabantoids, so let’s talk bettas. Of course, the domesticated bettas are appropriate for nano tanks, and they can add a splash of color and big fins, but they just don’t lend enough interest. Many of the wild-type betta species grow to 2½ inches (6.5 cm) or less, and are particularly well suited to life in a nano tank. You could choose from the members of the albimarginata, coccina, dimidiata, foerschi, picta, or splendens species complexes. I’m going to select one for inclusion here, and it’s a difficult choice, but in the end I think it has to be my favorite betta species, B. brownorum, commonly known as the scorpion or blue-spot betta.


The first consideration when maintaining any of the wild bettas is the cover. A tight-fitting cover is essential because they are all quite accomplished jumpers. The second consideration is water chemistry. This is a blackwater species that prefers soft, acidic water, though it will adapt to other water conditions.

Physical Characteristics

B. brownorum features a very intense red base color on both the fins and the body, but most photographs don’t show the color well. This species definitely looks better in person than in photos. There is usually a square blue blotch in the middle of the body, which is sometimes less present in females and sometimes lacking altogether.

Males have slightly longer fins, but sex determination can be difficult with this species. They can spawn as a bubblenester or as a mouthbrooder. No one is sure yet what determines the chosen spawning mode, and pairs will spawn one way, then the other, so the behavior is fascinating. When they spawn as bubblenesters, the nest will usually be submerged. They may choose a cave as a spawning site but seem to prefer it to be near the surface. Empty film canisters work well for this purpose, but they may be unsightly in your aquascape, so consider wrapping them in almond, oak, or beech leaves, or something similar to help them blend in.

Great for the Nano Aquarium

A pair of B. brownorum with a school of cyprinids can make a wonderful display in a nano aquarium. The presence of the cyprinids seems to encourage the bettas to spend more time in the open by functioning as an early warning system for the approach of potential predators.

Green Neon Tetra (Paracheirodon simulans)

The genus Paracheirodon includes three of the best fish for any size aquarium, but fortunately for us, they are all suitable for nano tanks too. Despite the fact that the cardinal tetra (P. axelrodi) is my favorite fish, and the neon tetra (P. innesi) is responsible for getting many people started in the hobby, I’m going to choose the green neon tetra (P. simulans) for this list.

Physical Characteristics

Green neons fall in the middle in terms of size, potentially reaching 1¼ inches (3 cm). Their distinctive stripe can either be blue or green, depending on the lighting and how the light reflects from the fish, which gives them an added level of interest. Below the green stripe, which extends the length of the fish, the red stripe is variable, sometimes extending to midbody and sometimes to the pectoral fins. This is the least common of the three species, but they can be obtained fairly easily, as they are regularly exported from Colombia.

Diptail Pencilfish (Nannostomus eques)

Diptail pencilfish are one of the most iconic of aquarium fishes. Their tail-down posture, slow swimming style (unless they have reason to move quickly), and tiny mouths are all interesting and combine to make one of the neatest packages in the aquarium hobby. Their habits also make them particularly well suited to keeping in smaller aquariums, where it is easier to feed them and less likely that any of their tankmates will intimidate or out-compete them.

Color Morphs

There are at least three distinct color morphs of Nannostomus eques, but it is entirely possible that these will eventually represent multiple species. The most commonly seen form sports a bright, light-colored horizontal stripe above a dark brown to black stripe that extends into the caudal and includes the lower lobe of the caudal fin. The fish seems to be fairly widespread, with most specimens reaching the hobby from Peru or from Manaus in Brazil. A second color form, inhabiting areas of Colombia, has a reddish tinge with a red lower lobe of the tail.

The third form hails from eastern Brazil, where it is typically exported from Belem. This fish features narrow blue horizontal lines and is really quite beautiful. Despite growing a bit larger than the other species included here, with individuals reaching 2 inches (5 cm) or slightly more in length, they’re still a wonderful nano aquarium addition. Watching a school cavort among thickets of plants is a sight one will not soon forget.

Small Species

There are a number of smaller species of pencilfish that are also great choices for nano aquariums, including N. anduzei, N. digrammus, N. marginatus, N. minimus, N. mortenthaleri, N. nitidus, and N. rubrocaudatus.

Chili Rasbora (Boraras brigittae)

This list could easily have been dominated by the genus Boraras, with all its members being tiny living jewels that fit well into most nano aquariums. Limiting myself to one member of the genus means that I must choose the fish I believe to be the most beautiful and striking of the genus, B. brigittae. This fish has many common names, including chili rasbora, mosquito rasbora, and Brigitte rasbora. Whichever one is used, the name will be longer than the fish, which does not reach ¾ inch (2 cm).

Physical Characteristics

The most prominent features are the teardrop-shaped horizontal markings surrounded by electric orange. The males are more colorful, while females tend to be larger and more robust. Pound for pound, or more accurately, milligram for milligram, this just might be the best aquarium fish we have. The colors are breathtakingly beautiful, the fish are completely peaceful, and they do great in a planted aquarium. Despite occurring in blackwater in nature, they are highly adaptable and will do well in almost any water conditions in the aquarium. When purchasing this species, don’t be fooled by the imitations.

B. urophthalmoides is frequently sold as B. brigittae. It sports a similar black marking, but it does not have any of the orange that makes B. brigittae so distinctive. Sometimes, mixed groups of B. brigittae and B. merah will be sold as B. brigittae.

Going Nano

There is a fantastic array of fish that will do well in nano aquariums, and it’s pretty hard to go wrong if you choose the species discussed here. But don’t limit yourself to this list. Just make sure that you understand the needs of the species you select and that you can care for them before you bring them home. If you haven’t accepted the challenges of a nano tank yet, what are you waiting for?

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