Marvelous Types of Minnows for Nano Tanks (And Beyond!)

Mike Hellweg

With the rise in popularity of both aquascaping and nano tanks over the past few years, small fishes that won’t harm plants, look great, and do well in small tanks have also become more popular. But I have been interested in different types of minnows and other small fish since long before the term “nano tank” was even in use, for purely practical reasons.

My first real fishroom was a spare bedroom on the second floor of our apartment. The landlord said that I could have as many tanks as I wanted in the room as long as they didn’t weigh more in total than a king size waterbed. That worked out to either a couple of bigger tanks, or lots of smaller tanks.

I went for the latter: a rack of tanks, a stand with a couple more, and a single larger tank against the opposite wall for my wife’s beloved oranda goldfish. With many smaller tanks, I had to concentrate on smaller species of fish, and my interest in tiny fishes began. Enter the cyprinids.

Mini Minnows

The family Cyprinidae, the fish we collectively call minnows, is far and away the most species-rich family of freshwater fishes. Some of the most popular aquarium fish are cyprinids, including goldfish, barbs, danios, and rasboras. And it’s an incredibly diverse group, ranging from the diminutive Paedocypris progenetica, which can lay across your fingertip without reaching either side, to the rather substantial mahseer, which is as long as an adult human is tall! Many of the most popular types of minnows for aquariums run to the smaller side, but our focus here is on the smallest of the small, what we call “nano fish.” 

While most of the smaller cyprinids will do just fine in small groups in a nano tank, they are also great to keep in a larger setup. Over the years I’ve learned by watching the residents of my tanks that the miniature cyprinids actually have fairly complex social structures and behaviors, but to see these at their fullest you need to give them a bit of space.

Long, shallow tanks are best, giving them plenty of room to exhibit their best behaviors. A shallow tank doesn’t need super high-end lighting to provide adequate lighting for plants even at the bottom, and a sponge filter, or small hang-on-back or canister filter, is perfect for filtration. Most of the plants I use in tanks like this—Cryptocoryne, Anubias, Bucephalandra, Microsorum, Vallisneria, and similar low-tech plants—don’t even need added CO2 to thrive. A few pieces of driftwood and a handful of dried hardwood leaves complete the picture. 

Since they are at the bottom of the food chain, small cyprinids in general do best in single-species groups, as opposed to a mixed-species community. They seem to feel their best with plenty of cover and others of their own kind around. The added bonus is that when kept in such groups they will show their best colors, which can be spectacular, and their best behaviors, which can be amazing.

They are not all schooling fish. Males of many species of cyprinids will actually stake out territories and stage mock combat while courting the groups of females that often swim about in small groups, checking out the boys. In fact, males of several genera have an internal structure composed of the base of the pectoral fin and a modified rib that allows the males to vocalize with little squeaks or chirps while sparring and courting.

While one might think that getting these diminutive beauties to spawn and raising their fry might be a challenge, nothing could be farther from the truth. In a well-aquascaped single-species tank where they feel happy, they will spawn regularly, sometimes daily, and occasionally juveniles will just appear in the tank with no work on the part of their keeper. 

Some species, like the popular celestial pearl danio (Celestichthys margaritatus), are downright precocious spawners. In a grow-out tank full of fry, with no adults present, after they reach about 8 to 10 weeks of age but before they take on anywhere near adult size and coloration, eggs and newly hatched fry will begin to appear.

Let’s have a look at the choices you may (or may not) find available for your own miniature minnow setup. 

Boraras: The Pygmy Rasboras

The genus Boraras is a group of six known species that are often available in the hobby, the chili rasbora (Boraras brigittae) being the most popular. Males of most species are bright red or have bright orange or red on their bodies. They have a complex social structure that can be seen in a large group. When wood or hardwood leaves are present, fry often appear without any interaction on the part of their keeper.

The Small Danios

celestial pearl danio

There are six species of what we collectively call danios that qualify as nano fish. Three of these are contained in the genus Celestichthys. The emerald dwarf danio (Celestichthys erythromicron) has a pattern of deep green stripes on an orange background. The glowlight danio (C. choprae) is incredibly popular and brightly colored, with blue bars against an orange stripe.

The most popular member is C. margaritatus, the celestial pearl danio. Unfortunately, they are quite shy, even in planted tanks. Adding another of the small danios, the gold ring danio (Brachydanio tinwini), causes them to overcome their shyness for some reason. This doesn’t seem to work with any of the other species. 

Lastly, the spotted or cheetah danio (B. nigrofasciata) has been in the hobby for decades. It is peaceful, and unlike the other species, not super active. The panther danio (B. aesculapii) is a recently introduced species where the males flash a lemon-yellow color when courting or sparring.

The genus Microrasbora once served as a catch-all for many of the small cyprinids. After recent revisions, two species remain, though only one, Microrasbora rubescens, is common in the hobby, as is at least one additional undescribed species. They sort of remind me of cardinal tetras in their pattern, save that where the cardinal is blue, they are a pale pinkish orange and where the cardinal is red, they are a rusty color. 

Microrasbora are hard-water fish that are extremely prolific. Both sexes are marked with the “Microrasbora spot,” a small black spot on the abdomen just over the first ray of the anal fin. Though similar in appearance, M. rubescens and the undescribed Microrasbora occasionally found in the trade should not be mixed in the same tank, as they may hybridize.

The genus Microdevario is currently made up of three diminutive species that are deep bodied like their much larger namesakes, the Devario. The most commonly offered species is the green neon rasbora (Microdevario kubotai). Other members of the genus are also often sold under this name. All are easy to care for, spawn readily, and the fry are easy to raise.

Danionella: The Glass Danios
These nearly transparent fish (with a touch of color, depending on the species) are very thin and very delicate. It is best not to handle them with nets, as this can cause serious injuries. Most of the known Danionella species are marked with a red spot on their flanks that is actually their spleen showing through. They do best on a diet of tiny live foods, such as microworms or vinegar eels. I have not had any of these species yet take flakes.

Paedocypris: The Infant Danios
The male Paedocypris progenetica is currently the smallest known cyprinid and one of the smallest known vertebrates, topping out at less than one centimeter! They have incomplete bone structures, including the skull, which remains unfused throughout their short lives—their brain is actually visible through the top of the head. 

Surprisingly these little guys are pretty hardy and spawn readily, laying their eggs on the underside of surfaces. Unfortunately, they only live for about 10 months, so if you can get them, it is best to induce spawning quickly. They do very well on a diet of microworms and similar tiny live foods, and mostly avoid non-living foods. Adults ignore fry, so all can be kept in the same tank.

Sundadanio: The Squeaker Rasboras
For years, we had a single species of Sundadanio, S. axelrodi, with several different color variants. Recently, we’ve found that these color variants are separate species, and the actual S. axelrodi is restricted to a very small portion of the range, from which it is rarely, if ever, collected. 

Males of all the aquarium species have black or red anal fins, while those of females are clear. They develop breeding tubercles on their face and cheeks, and along with their modified squeaker apparatus previously described, they spar and fight pretty viciously over females.

Sawbwa resplendens: The Asian Rummynose
Found in Lake Inle in Myanmar (Burma), Sawbwa resplendens actually prefers hard, alkaline water. Colors fade when they are not happy. Males are sky blue with a bright red nose and two bright red ovals on the caudal fin. Females are a pale yellow/gold color. They are surface spawners that prefer to place their eggs in floating plants like Riccia.

Tanichthys: The White Clouds
This group of four or more species is found along the east coast of Asia from southern China to Vietnam. It is possible that every river valley system has its own Tanichthys species, as new ones are being described all the time. 

Tanichthys spp. are the largest of the miniature cyprinids. They are easy to care for and easy to breed, and at least one species, the common white cloud (Tanichthys albonubes), has been bred into many variants and is one of the most common and popular aquarium fish.

Trigonostigma: The Harlequins

lamb chop rasbora

This group of five species is very popular in the hobby. Often two or even three of the species can be found in the same shop at the same time. They prefer softer and slightly acidic water, but don’t need extremes. They are popular community tank fish and have been in the hobby for decades. They are well known for laying their eggs on the underside of plant leaves. I’ve even seen them spawning in pet shop tanks!

Aquarium Care

One of the most important things to remember is that all these fish have tiny mouths, so foods that suit most community fishes will be too large for them. I’ve found all the species that I’ve kept so far do best with a mixed diet of tiny things. 

I feed newly hatched brine shrimp daily and add microworms a few times a week. In addition, I’ve found that finely ground pellet foods are readily taken by most species. To feed these, I put the pellets in an old pepper mill and grind out a meal right into the tank a couple times a week. 

Recently, manufacturers have been catching up to the nano fish craze, and I’ve found the newly available freeze-dried copepods are eagerly taken by all the miniature cyprinids that I’ve been working with lately. Most will also take finely ground flakes or the fine particles that wind up on the bottom of the flake food container, as these are already bite sized. I add powdered spirulina flake food a couple times a week to round out their diet. 

While it might seem like the miniature cyprinids would be delicate and will need super soft, acidic water, many of them—like some of the Danionella, Celestichthys, Sawbwa, Microrasbora, Microdevario, and the smaller Brachydanio—actually come from moderately hard, alkaline water and will do just fine in most tap water. Some also prefer cooler water, such as the popular celestial pearl danio, which comes from hard, alkaline ponds up in the mountains, so a heater isn’t even necessary for them. Many of the small danios and the white clouds are so hardy that they are actually among the first fish recommended for aquarists, and they are so spectacular many long-time hobbyists still keep them, including your humble author.

Other small cyprinids, like Boraras, Sundadanio, Paedocypris, and Trigonostigma, come from swampy acidic to very acidic water with almost no measurable carbonate hardness at all. They adapt to these environments by remaining very tiny, and in response to the lack of carbonates in the water, they often have incomplete skeletons even as adults, often with as many as 44 bone structures (skulls, ribs, vertebrae) that are incomplete. 

Keep that in mind when setting up their tank. It’s best to use reverse osmosis (RO) water, with tannins added from hardwood leaves, driftwood, or other botanicals. Many times, plants will not do well in water like this, so aquascaping with hardscape is actually more representative of their native biotopes.

A Bounty of Great Little Fish

Whether you are stocking a nano tank or a midsize setup, the mini types of minnows provide a bounty of great-looking, attractive little stocking options. And while they are small enough to thrive in nano tanks, providing them a long, shallow midsize aquarium can really help bring out their best behaviors. 

Whichever setup and type you choose, just be sure their few basic needs are met, provide them plenty of cover and others of their own kind to swim with, and you will be rewarded with a colony of amazing little fish that will show off spectacular colors and interesting behaviors. 

They are hardy and surprisingly long lived (with the exception of the Danionella and Paedocypris species), and you can enjoy watching them, and even observe their spawning behavior over several generations, all in a smaller tank right in your living room.

As always, don’t forget to sit, watch, and appreciate your fish!

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