20 Smallest Freshwater Aquarium Micro Fish That STAY Small

You have a small or medium-sized aquarium in your office or at home and wonder how to make it more lively.

This is easy: I have a couple of options on my mind and while these are nano species of schooling fish that are beautiful to observe they also fit the bill perfectly.

But what are the best freshwater inhabitants for a tiny 3.5 to 5-gallon tank?

This is not a lot of swimming space so I tried to list species that generally remain under 1 inch in length.

I finally decided to write about this because for some time now many of the new fish-keepers that read my blog have been asking me about the right micro pet fish for a freshwater aquarium.

Today I want to respond to these and a couple of similar queries or concerns you may have.

For instance, are there cold-water alternatives to the tropical species that suit a compact aquarium?

What are the types of small fish that remain the same size in their full-grown state?

I’ve compiled a list of the smallest freshwater nano fish that can live in a tiny tank based on my personal experience and some research I did.

Let’s dig in.

Right before we start, I will share a photo of one of my all-time favorite fish for small aquariums, the Celestial Pearl Danio (or also known as Galaxy Rasbora):

by mrglass1024

20 Tiniest Freshwater Aquarium Micro Fish

I took considerable time off to do some deep research on nano fish that would best suit a freshwater aquarium.

I combined that with my personal experience in fish keeping and I made sure to not list fish that get over 2.5 inches in full-grown length.

Here are the 20 smallest freshwater aquarium fish that are perfect for a nano tank:

1. Indonesian Superdwarf fish – Paedocypris Progenetica

by Humunuku

Maximum Size:	0.41 inches (1.04 cm)
Suggested Tank Size:	2 gallons for a small group
Suggested Water Temperature:	80.3 to 89.6°F (26.8 to 32°C)
Suggested pH range:	3 to 3.8

The Indonesian Superdwarf fish from the Paedocypris genus is a titleholder of sorts.

It was the smallest known vertebrate until the tiniest frog (Paedophryne amanuensis, discovered in the rain forests of New Guinea) leaped past in 2012.

An adult female Paedocypris Progenetica grows to a maximum of 0.41 inches (1.04 cm) only, while the mature males are mere 0.39 inches (0.99 cm).

To put things into perspective, an average adult specimen is the size of an adult mosquito! Seven Indonesian Superdwarf fish can thus comfortably shoal in a 3-gallon tank.

That being said, Paedocypris Progenetica still holds a world-record title: it is currently the smallest freshwater fish in the world that can live in home aquariums.

In fact, it still remains the smallest nano fish species in the world overall discovered to date.

Paedocypris Progenetica has a thin and somewhat transparent body with rather weird pelvic fins that have clutching pads that males use to grab the females during mating.

It is considered the cousin of the Paedomorphic Cyprinid fish family because they share similar features like an unprotected brain (their narrow frontals are the reason for this).

If you get your hands on Paedocypris Progenetica know that it’s a freshwater fish that likes its aquarium water acidic.

It hails from the black-tea swamps in Southeast Asia (the Indonesian island of Sumatra, to be specific).

These waters are generally more acidic than rainwater; they have a pH 3 acidity mainly due to the decomposing tree leaves inside that also give the swamps the dark tea color.

Therefore the Indonesian Superdwarf fish are a perfect addition to a 3 to 5-gallon blackwater planted tank.

This species is now facing an extinction threat due to habitat destruction in its ecosystem in the wild.

Suggested aquarium gear for a school of 15 Indonesian Superdwarf fish:

• Tank: Fluval Spec V 5-gallon aquarium kit (20.5 inches of horizontal length, comes with LED lights that can support beginner aquarium plants, and has a filter)
• Heater for tropical water conditions: This small and strong heater will do
• Indian Almond Leaves: they will darken the water and lower the PH
• Driftwood will also be beneficial to this aquarium setup as it lowers the water’s PH even more, over time
• Substrate: I can recommend this Aquasoil which will help with growing lush aquarium plants and will also significantly lower the PH in the water (you can also check more aquarium soil options here)

2. Pygmy Hatchetfish – Carnegiella myersi

by OpenCage

Maximum Size:	1 inch (2.54 cm) but they’ll generally remain around 0.8 inches (2 cm)
Suggested Tank Size:	10 gallons
Suggested Water Temperature:	74.3 to 79.7°F (23.5 to 26.5°C)
Suggested pH range:	4.8 to 6.4

Coming from the Carnegiella genus, the Pygmy Hatchetfish are gracious and timid schooling micro fish. They are so peaceful that they can easily co-exist with other animals in a fish tank as long as the tank mates do not harass the Hatchetfish.

This fish definitely needs equally timid aquarium companions, such as Loricariid catfish and Corydoras.

The Pygmy Hatchetfish is silvery in appearance and originates from the Amazon River basin in Peru.

The Pygmy Hatchetfish are among the best community freshwater aquarium fish that stay small throughout their lifespan. They grow to a maximum of 1 inch thus remain small at their maximum size and length.

They love swimming around the top of the aquarium, which makes them surface dwellers.

Therefore, you should ensure there’s a lid over the aquarium because your Pygmy Hatchetfish might try to jump out.

A 10-gallon aquarium is okay for these guys because they won’t take full advantage of its depth.

This implies that more water surface area is needed for their habitat, even though they don’t grow large in body size.

They need to be kept in a school of at least 6 companions to fully enjoy shoaling.

Adding floating plants such as Mosquito Fern or Red Root Floaters is also an excellent option for the Pygmy Hatchetfish. Also, ensure the water is still so the fish can enjoy moments of calm.

In my observation, high water current tends to stress Pygmy Hatchetfish.

They also prefer dim lights, which can be easily achieved if you follow my advice of adding a chunk of floating freshwater plants in the aquarium. Visit the link to skim my article on good options for that.

Pygmy Hatchetfish is an insectivore, it eats small insects on the water surface.

You can alternatively prepare bloodworms or small dried insects for them.

Feed them once or twice a day.

The ideal water temperature for this freshwater nano fish is 23°C while Ph should be at 5 to 6.

Suggested aquarium gear for a school of 6 Pygmy Hatchetfish:

• Lights: If you go ahead and add floating plants, then check these aquarium lights. These grow lights are perfect for aquariums with floating plants and some low-demand plants on the bottom of the tank.
• Filtration: For a small tank with 6 Pygmy Hatchetfish I recommend using this Sponge Filter because it is super quiet and yet – efficient. Supply it with a small air pump and connect the two with airline tubing: Check the Air pump here and also this Airline tubing.
• Heater: If you have a 10-gallon tank with a school of Pygmy Hatchetfish, then this heater
  will do a fantastic job at keeping the preset temperature, without exceeding it.
• PH Regulation: This nano fish species require low PH in its water, so if you are looking for an easy and natural way to achieve this, add some Almond leaves to the aquarium.

3. Stiphodon Goby – Stiphodon sp.

by Mike Burgh

Maximum Size:	around 1.5 inches (3.8 cm)
Suggested Tank Size:	10 gallons or more
Suggested Water Temperature:	68 to 77°F (20 to 25°C)
Suggested pH range:	6 to 7.5

The Neon Stiphodon Goby is a small freshwater fish that reaches no more than 1.5 inches or 3.8 centimeters upon maturity.

These species are among the members of the Stiphodon genus.

I do not recommend looking after these fish if you’re still a beginner in the hobby.

First of all the colorful Neon Stiphodon Goby is a herbivorous species of nano fish that should graze on algae and biofilm.

Offering any sort of meat to these fish may actually harm their digestive system.

To maintain a healthy diet for your Neon Stiphodon Goby you could simply get a product called Bacter AE, which will supply the aquarium with biofilm. Bacter AE introduces various sets of bacteria that produce large enough amounts of biofilm in the system.

You could also ramp up the lighting period of your aquarium lights to between 9 and 10 hours a day.

This encourages algae growth, which is another main source of food for the small Neon Stiphodon Goby. Having grow lights that are designed to support the growth of aquarium plants helps in this situation.

Do note that Black Algae will be the least appetizing type of algae for your Neon Stiphodon Goby. Another mandatory condition that should be met in order to look after the cool-looking Neon Stiphodon Goby is a well-oxygenated fish tank.

You should likely position an Air Stone in the opposite sides of the tank to achieve a sufficient oxygen content in the water. On top of good oxygenation, the Neon Stiphodon Goby also prefers a strong water flow.

If you decide on a strong canister filter for your aquarium filtration you could hit 2 birds with 1 stone. The canister filter will provide excellent filtration alongside a strong flow and water turnover.

For example, the Fluval 407 will deliver between 10 and 12-fold water turnover in a 20-gallon aquarium which is the recommended circulation rate for a fish tank with Neon Gobies. You may keep 1 male and 2 female Neon Stiphodon Gobies in a 20-gallon tank.

A 10-gallon tank can only support a single specimen.

If you could get a 20-gallon “Long” you could likely add another female to the group.

This nano fish does not tolerate aggressive tank mates and remains peaceful at all times.

The only time you may witness this fish becoming aggressive is when there’s a territory dispute between male Neon Gobies.

For this reason, I recommend keeping a single male with a small “harem” of females.

To sum up the recommended gear for a 20-gallon tank with a Neon Stiphodon Goby:

• Lights: The BeamsWork FSPEC Pent aquarium lights will absolutely keep your aquatic plants green and healthy. Ramp up the lighting period to about 10 hours a day if you need to stimulate more algae growth for the Stiphodon Goby to feast on.
• Filter: The Fluval 407 Canister Filter.
• Heater: With its slick design and accurate temperature maintenance, the Cobalt Aquatics Neo-Therm heater deserves your attention. You can check it at Amazon.
• Oxygenation: For a well oxygenated water you can use this Airstone and connect it via airline tubing to this small air pump (all links to Amazon).

4. Harlequin Rasbora – Trigonostigma heteromorpha

By PhilTheGouramiAndCo

Maximum Size:	2 inches (5 cm)
Suggested Tank Size:	at least 10 gallons, but preferably 20 gallons
Suggested Water Temperature:	70.7 to 84.2°F (21.5 to 29°C)
Suggested pH range:	5.8 to 7.7

The Harlequin Rasbora is the easiest nano fish to breed. All they require is a clean tank and regular execution of a proper diet.

The harlequins are also in high demand so you won’t have to tire yourself looking for them since many pet stores have them in stock.

Harlequin rasboras belong to the Trigonostigma genus and are so easy to keep that I actually listed them in my article on the easiest fish to take care of. If you’re a beginner in the hobby and want to have a low-maintenance home aquarium you should probably give that a read.

However, these nano pet fish are popular not because they are easy to maintain but due to their impressive schooling behaviors and vibrant colors.

They add great beauty and activities to the tank; their darker orange fins intensely change color and it is fun to watch, the least.

The change is triggered by various factors like the conditions of the tank and the level of stress they experience.

Socially, they remain at peace with other tankmates.

In fact, they are harmless to a fault as sometimes other bigger mates do take advantage of the Harlequins, turning them into regular meals.

Carefully choose which aquarium inhabitants these rasboras co-exist with.

I have seen them live comfortably with snails and shrimp.

Some reports confirm that Guppies and Platies can also do but I have not had the chance to put them together.

The Harlequin Rasbora generally grows up to 2 inches (5 centimeters) and keeps that as its standard body length.

A high variation of maximum size has not been observed for this fish.

Though Harlequin Rasboras are not the best for a small aquarium of 3.5 or 5 gallons they will perfectly suit a 10-gallon community fish tank.

In smaller fish tank sizes there’s not enough swimming space to guarantee a comfortable residence for a healthy number of schooling individuals.

Being omnivorous, Harlequin Rasboras dine on whatever comes their way, but they seem to enjoy plant detritus and insect eggs.

5. Dwarf Pea Puffer – Carinotetraodon travancoricus

by NickyG015

Maximum Size:	1.5 inches (3.8 cm)
Suggested Tank Size:	for a lone specimen – 10 gallons, but a heavily planted 5-gallon tank is also okay.
Suggested Water Temperature:	76 to 78°F (24.4 to 25.5°C)
Suggested pH range:	7.0 – 7.5

The Dwarf Pea Puffer from the Carinotetraodon genus is a small green fish with an insatiable appetite.

Given the chance, it can easily eat more than 6 blood worms per feeding and then beg for more.

This behavior often leads to overfeeding since many inexperienced fish keepers believe their puffers are still hungry. Unsurprisingly, the amount of waste these oddball nano fish produce is commensurate with their hunger.

This is why a single pea puffer requires between 5 and 10 gallons of free space, despite its minuscule size. Since they can be quite aggressive, these freshwater puffers aren’t the best fit for a nano community tank.

In some cases, they might even provoke or attack larger fish.

So if you’re going to keep them in a nano tank, it would be best to have them as solitary aquarium fish.

Fortunately, Dwarf pea puffers are one of the few fish that can live alone in a small tank.

Author’s note: When a Dwarf pea puffer feels threatened, it inflates itself by sucking in water. This increases its size and makes it harder to swallow by predators.

6. True Freshwater Bumblebee Goby – Brachygobius xanthomelas

by fifteenlostkeys

Maximum Size:	1.2 inches, but they usually stay under an inch (2.5 to 3 cm)
Suggested Tank Size:	10 gallons
Suggested Water Temperature:	71.6 to 82°F (22 to 28°C)
Suggested pH range:	7.5 to 8.5

The True Freshwater Bumblebee Goby is a micro fish that typically stays under an inch when fully grown. Because of this species’ somewhat territorial behavior, the minimum size for its tank should be 10 gallons.

The Bumblebee Goby is assigned to the Brachygobius genus.

True Freshwater Bumblebee Goby can be territorial and will try to fight off fish in its weight category.

Also, males will be especially territorial among each other.

However, I recommend getting at least six specimens.

This way the aggression will be spread among themselves, and the Bumblebee Goby group can be housed alongside other fish.

I would also recommend avoiding housing a True Freshwater Bumblebee Goby with any fish with longer fins such as goldfish, tetras, or smaller long-finned species.

A Bumblebee Goby that’s not in the mood may nip at its tank mates’ fins.

It’s not a risk worth taking, in my experience.

Author’s note: This fish will likely not accept commercial foods. You’ll need to provide it with either a live culture of micro bugs or frozen foods.

Anyway, I need to point out that you will never see this fish labeled as “True Freshwater Bumblebee Goby” in your local fish store. That’s because there are many fish species known as Bumblebee Goby and almost all of them actually inhabit brackish water.

Before putting this fish in a freshwater tank, always make sure it’s the right variety. Resorting to a reliable fish ID website such as SeriouslyFish is usually a good idea, when you’re not certain about a species.

Also, some of the more responsible online fish stores will have the True Freshwater Bumblebee Goby in stock. Visit this link to check some of these online fish stores.

7. Galaxy Rasbora – Danio margaritatus

By amanofscience

Maximum Size:	around 1 inch (2.5 cm)
Suggested Tank Size:	10 gallons and above
Suggested Water Temperature:	71°F – 78°F (21.6°C – 25.5°C)
Suggested pH range:	6.8 to 7.5

Coming from the Danio genus, the Galaxy Rasbora, also known as celestial Pearl Danio, is yet another tiny freshwater aquarium fish species.

I love it mostly for its cool-looking colors which, at times, seem too stunning to be real.

I’ve also listed this fish in my compilation posts of the most colorful and the coolest freshwater fish for aquariums. Visit the link to check those out as there are some really cool mentions there, worth knowing about.

Although the different sexes of Galaxy Rasboras have different colorations, they nevertheless share orange, white, or golden spots (sometimes strips) spread all over their dark blue or black body color.

They also have transparent gill plates through which you can see their blood vessels.

The Galaxy Rasbora has been considered as one of the best small cold water fish since its discovery in 2006.

It grows very small, up to 1 inch long for an adult specimen.

However, it has become a hot cake in the aquarium industry.

It is in such massive demand that its population now declines due to over-fishing or the destruction of its natural home.

For that reason, this fish faces extinction.

These are schooling fish, so I advise that you keep them in a group of about 6 specimens for a 10-gallon fish tank.

This way, they can shoal comfortably.

Also, put in plants to give them cover, for they are quite timid.

Set your heater at 23°C and the pH at 7.

Do not worry too much about bright light because they are used to direct sunlight in their natural habitat, but try to make it moderate if possible.

8. Gulf Coast Pygmy Sunfish – Elassoma gilberti

By michael sarr

Maximum Size:	1 inch (2.54 cm), but the majority remain at 0.98 inches (2.48 cm)
Suggested Tank Size:	5 gallons or more
Suggested Water Temperature:	68 to 74.3°F (20 to 23.5°C)
Suggested pH range:	6.5 to 7.5

The Gulf Coast Pygmy Sunfish is another small pet fish with a stunning appearance that you should consider for your home aquarium.

It is one of the seven species that make up the Elassoma genus.

This Pygmy Sunfish is the shortest of the species as it grows to a maximum of 1 inch (2.54 cm) long but the majority of specimens remain at 0.98 inches or 2.48 centimeters.

The other 6 species of sunfish range between 1.2 to 1.3 inches.

The Gulf Coast sunfish was discovered in 2009.

It has similar features to her cousin Elassoma Okefenokee and for a long time, I couldn’t quite tell which is which.

These dwarf fish are beautiful and easy to keep as long you maintain them in the right conditions. The dominant male is especially stunning to watch while coloring up to claim territory — they turn electric blue and black.

From what I have observed and gathered from other aquarists, these fish tend to demand live food like micro-worms and insects over prepared food. They will also predate and feed on small snails and snail babies.

This is critical as they are mainly wild-caught.

A densely planted aquarium with soft water is also necessary because it is a resemblance to their natural habitat in the wild.

Calm water will be greatly appreciated by the Gulf Coast Pygmy Sunfish.

The males put on intense and impressive colors, especially during the mating season (which is pretty much constantly after they become comfortable enough).

The fish is a survivor – it requires low oxygen, unlike some other fish.

It will acclimate to a wide range of water parameters and the only thing that’s required for it to spawn is a constant supply of live food.

A Ph of around 7-8 is okay.

You should set your water temperature at 23°C.

A 5-gallon aquarium is comfortable for a pair of Gulf Coast Pygmy Sunfish.

A 10-gallon tank can comfortably accommodate a modest school of this fish.

If you want to include other tankmates, consider a 20-gallon tank.

9. Dwarf Pencilfish – Nannostomus marginatus

By bimon22

Maximum Size:	0.98 inches (2.5 cm)
Suggested Tank Size:	a minimum of 10 gallons to house a school
Suggested Water Temperature:	73.4 to 82.4°F (23 to 28°C)
Suggested pH range:	5.7 to 7

The Dwarf Pencilfish is a member of the Nannostomus genus.

From what I have seen, the Dwarf Pencilfish are very active, especially in blackwater aquarium conditions where they tend to be more at home.

Lower the lights, though, as they are no fans of bright lighting.

You can use floating plants to absorb the lighting and create some shadowing.

The Dwarf Pencil also enjoys low currents so the filtration should be modest.

Of course, this calls for a 10-gallon tank at the very least.

I would also recommend that you only add them to a mature aquarium where stable water conditions can be guaranteed, as per my personal experience with these guys.

Typically, the Dwarf Pencilfish grow up to 0.98 inches (2.5 cm) which effectively classifies them as an aquarium type of fish that stays small even in their adulthood.

10. Dwarf Rasbora – Boraras maculatus

By Babsylicious

Maximum Size:	1 inch (2.54 cm), but most adult specimens will remain at around 0.75 inches (1.9 cm)
Suggested Tank Size:	5 gallons or more for a school
Suggested Water Temperature:	73.4 to 80.6°F (23 to 27°C)
Suggested pH range:	4.7 to 6.2

The dwarf “Boraras” rasbora is one of the strikingly attractive tiny fish on this list.

The Dwarf Rasbora is considered a member of the Boraras genus.

The males’ bright ruby red color stands out in the aquarium pretty much like a rainbow in the sky.

The Dwarf Rasbora has become trendy because of its entertaining schooling behavior and its peaceful disposition.

These qualities make it a fitting companion among other tank-mates.

This pretty dwarf aquarium pet fish grows to a maximum of only 1 inch (2.54 cm), but the average standard size for a single specimen would be around 0.75 inches (1.9 cm).

Such a size essentially makes the Dwarf Rasbora one of the smallest fish in the aquarium trade.

If you are not a fish expert, you could hardly differentiate it from its cousin, the Chili Rasbora.

I once heard some suspicious buyers whisper that the two are actually the same species from different environmental conditions.

I just chuckled.

The Boraras Maculatus thrives best on varied meat food: bloodworms (I suggest you chop them), micro-worms, etc.

You should also maintain the water clean and use floating plants to keep the light dim as this micro fish does not really like bright lighting.

The Dwarf Rasbora is the smallest schooling nano fish for home aquariums.

With a full size of 0.75 inches or 1.9 cm per specimen, these fish can comfortably school in a tiny 5-gallon tank.

However, they easily get stressed and distracted from shoaling when housed together with harassing tankmates.

So if you must keep them in a community tank, find agreeable tank mates like the dwarf shrimp.

Also, provide hiding places in the form of aquarium decoration or live plants just in case they are frightened (which is bound to happen once in a while).

Dwarf Rasboras prefer the middle or top level of a fish tank’s water column.

A 6-gallon fish tank is more than suitable for a small school of this micro fish species.

For the aquarium parameters, I recommend you set the heater at 25°C and maintain a pH of around 5 (acidic water).

11. Pristella Tetra – Pristella maxillaris

By mightaswelltroll

Maximum Size:	1.9 inches (4.8 cm) but most adults will remain at around 1.7 inches (4.3 cm)
Suggested Tank Size:	10 gallons at the absolute minimum, but preferably 15 gallons
Suggested Water Temperature:	74 to 82°F (23.3 to 27.7°C)
Suggested pH range:	6.2 to 7.5

Pristella Tetras are another tiny fish I wholeheartedly recommend for a beginner aquarist.

These small tetra fish belong to the Pristella genus.

I love watching these Amazon River natives as they have lovely round bodies and a black strip right across the dorsal fin. They honestly remind me of small sharks and are really the closest thing you could get at this size that would resemble a freshwater shark.

Anyway, Pristella tetras, unlike sharks, are very peaceful when being kept in a small school of, say, 6.

They possess somewhat transparent bodies for which they are sometimes called “X-ray” tetras.

However, Corydoras make perfect companions for them since both species are peace lovers.

They grow to be about 1.9 inches or 4.8 cm if they are kept under healthy conditions.

The smallest aquarium size I recommend for housing them is a 10-gallons of water for living space.

Pristella tetras are restless schooling fish so they need the swimming space.

On top of that, ensure there are rocks and plants and the current is moderate because they are very active explorers.

12. Rosy Loach – Tuberoschistura arakanensis

By Cinder1977

Maximum Size:	1.2 inches (around 3 cm)
Suggested Tank Size:	30 gallons because of how active these micro fish are in the aquarium
Suggested Water Temperature:	69 to 78°F (20.5 to 25.5°C)
Suggested pH range:	6.7 to 7.8

The Rosy Loaches are representatives of the Tuberoschistura genus.

First, I suspect this scientific name is fictitious, but let’s go with it.

Secondly, I have for five years had Rosy Loaches in one of my aquariums so I can speak from personal experience here.

What impressed me most when I just had these small fish was how fast they adapted to the new environment without any hitches.

Now, they are really charming and hyperactive, especially when in a large school.

By the way, all loach families should be kept in large groups if you don’t wish to see them withdraw.

This means keeping them in large groups of at least 10 specimens.

Because of their endless curiosity, your aquarium should have plenty of plants, driftwood, and rockwork to be lively for them.

Yet, you still have to leave enough clear space for shoaling, especially in mid-water as rosy loaches are middle to bottom aquarium dwellers. Hanging out on the bottom also means that you should be very careful if you want to pair them with other small bottom dwellers like the tiny pleco species. Most plecos are territorial, be they small or large.

I have noted that they rarely prey on snails though.

Maybe just my Rosy Loaches are nice to snails.

I am yet to find out if this is a universal phenomenon.

Maintain them with a fleshy diet supplemented with vegetables if you want to enjoy watching their vibrant orange coloration.

There was a time I would feed them mosquito larvae, and they loved it.

Another unique aspect I have observed with them is this constant power struggle.

Each of them wants to be the leader of the group.

However, this competition to be in charge rarely ends up in a fight.

This is a lesson we humans should emulate, maybe.

I have also observed that at other times some of them form distinct territories that are supposed to be no-go zones of sorts.

That’s how unusual some of their behavior can be.

I keep my Rosy Loaches at 77°F ( 25°C) temperature of the aquarium water.

Now, this is where you look for a 30-gallon tank.

As I noted, these are hyperactive pet fish.

I would say that the longest Rosy Loach I have seen in my aquarium measures about 1.2 inches or around 3 cm.

13. Least Killifish – Heterandria formosa

By JoeCamaro

Maximum Size:	1.2 inches (3 cm) but in general, these fish will remain at around 0.8 inches (2 cm)
Suggested Tank Size:	5 gallons and above
Suggested Water Temperature:	68 to 78°F (20 to 25.5°C)
Suggested pH range:	7.2 to 8.2

What’s in a name?

The irony here is that the Least Killifish doesn’t belong to the killifish family.

This small white aquarium fish is actually part of the Heterandria genus in the Poeciliidae family and is the 7th smallest fish in the world.

The male is around 0.8 inches while the female is 1.3 inches.

The Least killifish is also the smallest livebearer fish in the world discovered to date. Unlike other livebearers, which produce all the fry at once during the breeding, the female Least Killifish produces hers sparingly.

She brings forth about 3 fry babies every 5 to 10 days until she is finally done.

Her fry is bigger than the other livebearer fish’ fry.

This tiny freshwater aquarium fish is easy to care for because it easily adapts to various water conditions.

Again, it is peaceful and friendly by nature (its name has nothing to do with killing) so it can co-exist with other species.

Do not place alongside aggressive tank mates though.

The following setup will guarantee you excellent results: Ph of 7.6 to 7.8, a temperature of 75°F (24°C), and a 3 to 5-gallon tank at the least.

14. Pepper Corydora – Corydoras paleatus

By Crowbot18

Maximum Size:	2.8 inches (7.1 cm) but most will stay at 2.5 inches (6.4 cm)
Suggested Tank Size:	10 gallons at the very least
Suggested Water Temperature:	72 to 79°F (22.2 to 26.1°C)
Suggested pH range:	6.2 to 7.2

The Pepper Corydora is associated with the Corydoras genus of freshwater catfish.

Pepper Corydoras, sometimes called peppered cory catfish, are one very relaxed freshwater fish and are a good choice for micro aquariums.

In fact, it’s peaceful and calm to the point it appears lazy.

What would you call a fish that hardly swims to the top, and instead is content with scourging the substrate for food or, perhaps, for fun?

I’d call it a down-to-earth fish. No, a-down-to-substrate fish…

Well, it hovers along the tank bottom and if it sights some food there, it charges for it at once like a rhino.

Watching it “hunt” for food is part of the fun for me.

Okay, it does come to the surface to gulp in oxygen from time to time as it has the ability to do so through special organs.

This characteristic enables it to survive in low oxygen conditions.

Still, on peace, I should point out that Pepper Corydoras are among the few fish types that never get aggressive with each other during mating.

I kind of respect them for that.

This dwarf fish originates from Brazil and Uruguay, but I have heard unverified reports that it has natural habitats in many other South American countries.

The pepper cory grows to a maximum size of 2.2 inches.

A tank of at least 10 gallons is suitable for a pair of pepper cories.

Nevertheless, do not hesitate to use a 20-gallon tank.

In their natural homes, they feed on insects and plants so try to provide a similar diet for them in the aquarium.

The alternatives are regular live food and sinking pellets.

15. Dario Dario – Badis bengalensis

By kharma_chameleon_

Maximum Size:	0.78 inches (2 cm)
Suggested Tank Size:	5 gallons or more for a single male with 2 females
Suggested Water Temperature:	68 to 78.8°F (20 to 26°C)
Suggested pH range:	6.8 to 7.8

The Dario Dario is a small Indian fish or, to be more specific, a native of the Brahmaputra River.

This beautiful on the outside and timid by character fish comes from the Dario genus. An average adult is 0.8 inches so they can survive in a 5-gallon tank, or even less.

The male Dario Darios put on more coloration when wooing a female.

The most colorful male gets the lady.

It is found along the shallow shores that are dominated by vegetation.

So in your tank, put sand to mimic its natural habitat. Black sand is ideal as it makes the colors of our guys pop beautifully. Black Diamond Blasting sand is a safe and relatively cheap option.

Anyway, according to some research I did, Dario Dario might get aggressive and territorial.

Therefore, the tank you prepare for them should contain boundaries between them, especially males.

Put coconut shells to act as caves (read private territories) in the aquarium.

Driftwood is also a sound choice for decoration with these guys.

Again, they are not what I would call shoaling fish.

How do you shoal when you fight to the death over everything from food, mates to space?

But other times (rarely) they bury the hatchet and try to put up a show.

Please do not get the impression that the Dario Dario is by nature hot-headed; they are actually peaceful as long as there are no others like them to compete with.

16. Dracula Minnow – Danionella Dracula

By Peter Macguire

Maximum Size:	the largest adult ever measured was 0.65 inches (1.67 cm)
Suggested Tank Size:	5 gallons or more for a school of 10
Suggested Water Temperature:	66 to 78.8°F (19 to 26°C)
Suggested pH range:	6.5 to 7.5

Coming from the Danionella genus, the Danionella Dracula is also known as the Dracula Minnow.

This is one of the few very bizarre micro freshwater fish that I have seen.

Its appearance can inspire or has certainly inspired a horror film image.

This very small fish species was discovered in the pools in Myanmar, Burma, in April 2007.

An average Danionella Dracula adult is a mere 0.6 inches (1.5 cm) long.

If you are looking for some weird tiny fish to add to your beautiful freshwater tank as a contrast, go for this.

The horror features are their large eyes and unusual teeth.

Now the teeth are virtually fangs made of protruding bones sticking out from the jawbones.

Males use these scary fangs for sparring. Indeed, the jaws dominate the males’ heads.

Have I mentioned it is somewhat colorless?

You do not want to imagine what these dwarf fish would look like if it were the size of, say, an adult dinosaur.

Again, it is not much of a threat to other tank mates despite its scary teeth.

It is easy to care for. It’s a schooling species so do not buy just one.

I advise you to go for a dozen.

Feed them on nematode micro-worms, Tubifex, or Daphnia. If you want, you can try raising your own micro-worms at a “farm” which is a great way to feed fish. To start the nemotode farm you just need:

• a small plastic container
• some simple carb food like bread
• a starting culture (easy to find online)
• some yeast.

17. Chili Rasbora – Boraras brigittae

By Peter Macguire

Maximum Size:	0.78 inches (2 cm)
Suggested Tank Size:	5 gallons or more
Suggested Water Temperature:	75 to 82.4°F (23.9 to 28°C)
Suggested pH range:	4.5 to 6.8

The Chili Rasbora belongs to the Boraras genus, same as the Dwarf Rasbora.

It is sometimes called Mosquito Rasbora and is a peaceful fish you could catch from the Indonesian rivers.

Anyway, they are one of the best nano species of fish in terms of coloration, especially if you keep them in a larger group (like 10 or 12) where their confidence level is high.

An adult Chili Rasbora is about 0.78 inches (around 2 centimeters) and is either red or orange in color.

Chili rasboras will best fit a small tropical fish tank.

They remain small in size even in adulthood and can adjust to a great temperature range.

The Chili Rasbora fish like and will thrive in aquarium water that mimics the conditions of tropical swamps.

The water in the fish tank should have a higher temperature of around 78°F (25.6°C) and high acidity of, ideally, below 5.5 pH.

If you’re planning to house a school of these small red fish in a 5-gallon aquarium, I recommend checking my top picks for tiny water heaters. It also helps to put the Chili Rasbora fish together with tankmates of their size and temperament, such as dwarf Caridina shrimp, and pygmy Corydoras.

Chili Rasboras enjoy feeding on daphnia, Cyclops, crushed fakes, or mini-bloodworms.

Since they come from a blackwater habitat, you’ll do them a great favor if you include floating plants or Indian almond leaves in your aquarium.

The floaters will cast shadows and the Indian almond leaves will release tannins in the water, coloring it a darker tint, while supporting the Rasboras’ immune system.

My personal experience has been that the Chili Rasbora tends to be somewhat delicate and sensitive to swings in the aquarium water’s parameters.

Maintaining stable water chemistry is beneficial for the well-being of these fish.

Related: Learn more about how to take care of a Chili Rasbora fish.

18. White Cloud Mountain Minnow – Tanichthys albonubes

By CMReaperBob

Maximum Size:	1.4 inches which equals 3.6 centimeters
Suggested Tank Size:	5.5 gallons but a 10 gallon fish tank is preferable as the minimum
Suggested Water Temperature:	59 to 71°F (15 to 22°C)
Suggested pH range:	6.5 to 8.5

This micro fish species from the Tanichthys genus was first collected by a Chinese boy scout leader who promptly named its genus.

This happened near White Cloud Mountain in China.

For a better coloration display, you should put The White Cloud Minnow up against a dark substrate in a heavily planted aquarium.

White Cloud Mountain Minnows are most definitely the best nano fish for a cold water aquarium.

These small fish can tolerate temperatures as low as 41°F or 5°C in the wild.

These minnows will thrive in fish tanks that maintain a temperature range of 59 to around 71°F (15 to 22°C).

In fact, warmer temperature ranges would result in a short life and a weak immune system for the White Cloud Mountain Minnows.

They can also gradually adapt to hard aquarium conditions.

This mini fish is so peaceful that it would rather eat the leftovers or even nothing than compete for food.

Its docile behavior, however, shouldn’t fool you.

If you do not shut the top of your aquarium, this seemingly timid tiny fish would sometimes jump out to its inevitable death.

They enjoy feeding on insect larvae, shrimp, or insects.

You can also feed them on flakes.

Though I said they could tolerate hard aquarium conditions, I do not encourage you to stress them just because they are survivors.

Your aquarium size shouldn’t be less than 5 gallons.

The longest White Cloud Minnow I have seen was about 1.4 inches, and I do not think they grow beyond that.

Otherwise, the fish is easy to maintain and I would recommend it for a beginner who is yet to fully comprehend the wonders of fish-keeping.

19. Ember Tetra – Hyphessobrycon amandae

By itstherussianmafia

Maximum Size:	0.9 inches (2.3 cm) but a standard length of a fully-grown specimen is around 0.7 inches (1.8 cm)
Suggested Tank Size:	10 gallons or more, because of how active Embers are
Suggested Water Temperature:	71.6 to 83.3°F (22 to 28.5°C)
Suggested pH range:	5 to 7

The Ember Tetra is a member of the Hyphessobrycon genus, the species of which are generally known as tetras.

If you are going to keep Ember Tetras, prepare an aquarium with logs, driftwood, and plants.

This is because their natural habitat is green and forested and much of that ends up in the riverbed.

These rivers flow slowly and smoothly.

Coming from such a habitat, the Ember Tetras are cool small fish that have learned to be fast swimmers and shoal around the middle level of the tank.

Ensure the filter is quiet to imitate the smooth-flowing river.

They are not timid, although you need to give them time to adapt to a new aquarium.

Throughout that period, they may act overcautious.

With a full-grown maximum size of 0.7 inches (around 1.8 cm), the Ember Tetras are considered to be the smallest tetra fish species.

On rare occasions an adult may reach 0.9 inches, however, this is seldom recorded even with excellent care.

Though tiny, these fish are easy to recognize for they are red in color, with orange-rimmed eyes.

They look way more beautiful and attractive when kept in larger groups.

Feed them regularly on high-quality pellet food.

A pH of 6.5 is good enough.

I recommend a 10-gallon tank and a temperature of 26°C.

20. Orange Guppy – Poecilia reticulata

by Yas400

Maximum Size:	varies with types but usually no more than 1.4 inches (3.6 cm)
Suggested Tank Size:	5 gallons but larger tanks are better for groups and management of fry
Suggested Water Temperature:	72 to 82°F (22 to 28°C)
Suggested pH range:	6.8 to 7.8

The Orange Guppy is one of the many breeds of the common guppy fish from the Poecilia genus.

This tiny orange fish is exceptionally hardy and can survive staggering temperature drops down to 55°F. Due to this, it is often cited as one of the best small pond fish.

However, in order to thrive, it should be kept in a temperature range of 72°F to 82°F.

Apart from its hardiness, the Orange guppy fish boasts a magnificent fantail which exudes an almost regal elegance during swimming.

If you’ve never kept guppies before, you should keep in mind that they breed extremely fast.

So if you keep a group of them in a small 5-gallon tank, they will populate it quite fast unless you export the fry.

The Best Species for A Small 3.5-Gallon Fish Tank

If you are going to use a small gallon count fish tank, you need to make sure you’ve found out the right minute fish that can live comfortably in it.

However, I have taken the time to do your homework for you.

These are the best micro fish for a 3.5-gallon tank:

• Indonesian Superdwarf fish
• a single Dario Dario
• Dracula minnow
• Male Least killifish
• Dwarf rasboras

Top Fish Selections for a 5 To 5.5 Gallon Aquarium

If you have space for a micro aquarium of 5 gallons and above, you should go for it (link to Amazon, to check some good tank options). I have taken the time to gather and highlight the right sections for your convenience.

That being said, here are some of the best micro fish for a small 5-gallon tank:

• 15 Indonesian Superdwarf fish
• 3 Dario Dario
• 3 Gulf Coast Pygmy Sunfish
• between 6 and 10 Danionella Dracula
• 4 pygmy corydoras
• 10 Dwarf rasboras
• 12 Chili Rasboras
• 7 Least killifish

Something You Should Know About Looking After Nano Pet Fish

Small fish tanks may occupy less space, but they come with some serious inconveniences. For one, water is polluted too soon in such confined spaces.

The decomposing wastes raise ammonia or nitrites concentrations, which are both extremely toxic to live fish. There could also be a fluctuation in Ph (the water’s acidity), which again is extremely stressful and potentially lethal to aquarium inhabitants.

All of these volatile metrics may lead to an unhealthy tank environment for your new nano fish. You would have to change the water more often, and this too comes with its complications. Visit the link to see an explanation of how accidentally changing too much water in a small tank can stress its fish residents.

On the other hand, a big fish tank is ideal.

Larger volumes of water are easier to maintain, and the aquarium’s water parameters remain stable once you have things set up.

Another thing to consider is that it’s well known that when fish feel confined in smaller aquariums they become more aggressive. Bear this in mind when you are deciding on your fish tank’s size.

I’m not saying it’s impossible to achieve.

I’m saying that if you have the means – get the larger tank and all necessary side equipment. The ball is in your court.

My Final Thoughts

If you have any questions, feel free to post them in the comments section.

It was not an easy task to collect this information from all over the place.

It took me about 5 days of work to Aquanswer your concerns about relevant species for a thriving nano fish tank.

I did some heavy research in order to come up with the freshwater micro fish that are among the smallest ones in the aquarium hobby, even in their adult stage of life.

Mind that some of these will not be an easy find, but if you manage to get your hands on them – it’s absolutely worth it.

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