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17.9. Nano Aquarium

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Author : David Bogert

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Time To Read :
9 minutes
Difficulty : Level 6

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Small aquariums are called nano aquariums and are easy to set up.
“Nano aquariums” are simply small aquariums. There are very roughly two different “nano aquariums” in the hobby. One is under five gallons (20 liter) in size and one is from five to fifteen gallons (20-60 liter). They will be treated separately.

Under Five Gallon (20 liter) “Nano” Aquarium

One special situation is when someone has a small one half to five gallon (2-20 liter) aquarium and wants to add some colorful fish. They need something called “nano” fish which do not require schooling. Such fish include Endler livebearer, scarlet badis (Dario Dario), pygmy sunfish, dwarf chain loaches, peacock gudgeon, sparkling gourami, Tanganyikan shellies, and the betta. My personal favorites are the Tanganyikan shellies (Lamprologus ocellatus and their relatives). They are very small cichlids with great personalities.

Lamprologus ocellatus Gold Shellie
Lamprologus ocellatus – Gold Shellie

Most beginners will stock a male betta in their nano tank. This is probably the best fish for any beginner with a small tank. But note bettas need to be alone in a small tank. Contrary to well meaning but erroneous popular opinion, bettas do just fine in very small tanks. I like to have enough room for a small filter but other than that one can go very small with bettas quite successfully. And bettas do NOT require a heater in most homes which stay above 67°F (19.5°C). For more on bettas, go to this link:

17.7. Aquarium Bettas

Danio choprai Glowlight Danio
Danio choprai – Glowlight Danio

Guppies also seem to be a favorite but I have had some big problems with guppies. They are very highly inbred and tend to have a lot of diseases. I can’t keep a male guppy alive for more then two weeks. I like the Endler livebearers (some say these are just guppies with a different name) as they are not as touchy as modern guppy strains.

Poecilia wingei Endler Livebearer
Poecilia wingei – Endler Livebearer

Five to Fifteen Gallon (20-60 liter) Nano Aquarium

If one has a five to fifteen gallon (20-60 liter) one can stock either the single fish listed above or one can easily stock various small schooling nano fish. A good option is schools of six or more Daisy’s blue ricefish, white cloud mountain minnows, otocinclus, pygmy corydoras, Celestichthys margaritatus (“celestial pearl danio” or “galaxy rasbora”), glowlight tetra, dwarf pencilfish, various small danios, small barbs such as ruby, and cherry, and small rasboras such as emerald, dwarf, lampeye, chili, phoenix, somphong, galaxy and pygmy. A school of green kubotai rasbora is stunning!

Look up the adult size of the fish. Some experienced nano fish enthusiasts are of the OPINION that, if a school of fish is needed for a small tank, one needs to stick with fish under one inch (2.5cm) as adults. But this is only an OPINION.

Corydoras pygmaeus, Pigmy Corydora
Corydoras pygmaeus – Pigmy Corydora

The list of fish which get larger than one inch includes such fish as the harlequin rasbora (2 inch or 5cm), pork chop rasbora (2 inch or 5cm), golden barbs (4 inchesor 10cm), Odessa barbs (3 inch or 7.5cm), checkerboard barbs (2 inch), most corydoras (3 inch), black phantom tetras (2 inch), lemon tetra (2 inch), bleeding heart tetra (3 inch), tiger barbs (3 inch), blue-eyed rainbowfish (pseudomugil) (2 inches) and most of the glofish (the danios are fine). Hobbyists will be surprised how big these fish get. They MAY be cramped in small tanks.

But , again, this is an OPINION, not a scientific fact. So if you want a school of harlequin rasboras in a ten gallon (40 liter), go for it!  I’ve seen a large school of two inch (5 cm) black phantom tetras in a ten gallon (40 liter) and they were doing just fine. Many have large numbers of tetra and barb glofish (actually genetically modified albino black phantom tetras and tiger barbs) in a ten gallon. (40 liter) Others freely use ALL corydoras in small tanks.

Kubotai Neon Green Rasbora
Kubotai – Neon Green Rasbora

Schooling fish MUST be in schools of at least six and preferably ten fish. This is not to keep them “happy”. This requirement is because these fish CAN become very aggressive if not in a school. Groups of one, two or three fish of species like tiger barbs, serpae tetras, black phantom tetras, zebra danios and most glofish (bettas and sharks being the exceptions) have become killers of other species of fish. So only stock smaller numbers of these schooling fish in single species aquariums.

Puntius titteya Cherry Barb
Puntius titteya – Cherry Barb

Surprisingly even three-inch or four inch (7,5 of 10cm) fish like platies and molly’s can be put into a small 5 to 15 gallon (20-60 liter) tank with no adverse effects. What is important is not to mix the size of fish in a small tank. The smaller fish naturally want to stay away from the larger fish, and they can’t do that in a small aquarium. This will stress the smaller fish out and cause it’s early demise. So always stock fish of a similar adult size in a smaller aquarium.

Dario dario, Scarlet Badis
Dario dario – Scarlet Badis

All Nano Aquariums

Nano aquariums are like all aquariums in that it is best to decide on what one wants in an aquarium early on. You can have a planted aquarium with no aeration and one or two fish OR you can have a fish aquarium with a lot of fish and a few very hardy plants that can grow in aquariums with aeration (Java fern, Java moss, anubias, swords). Even moderate numbers of fish typically won’t make it with plants. Either lack of aeration will kill the fish or aeration will kill the plants.

I find that the best situation with a small tank is obtained by using only one species of fish in a single aquarium. In a small tank there can easily be aggression issues and determining “compatibility” of different species of fish is always a crap shoot in any case. So if one’s aquarium is smaller than fifteen gallons (60 liter), it is recommended one only stock one species of fish. If one wants different colors that are a large number of color variations in the Endler livebearer. And the very easy platy has dozens of color variations.

Pseudomugil luminatus Red Blue-eyed Rainbowfish
Pseudomugil luminatus – Red Blue-eyed Rainbowfish

The various Neocaridina shrimp are somewhat tricky and should be avoided by the beginner. Blackwater fish like neons, cardinals, rummy noses, apistogramma cichlids and rams should also be avoided by beginners. Blackwater fish require well established, large filters that give crystal clear water. Some people have done nano tanks with bumblebee goby but these are somewhat tricky fish.

Danio erythromicron Emerald Dwarf Rasboras
Danio erythromicron – Emerald Dwarf Rasboras

Male bettas in any tank under 15 gallons (60 liter) need to be by themselves. They CAN sometimes attack other fish in a small tank (some bettas are peaceful, some are very aggressive. It is a bit of a crap shoot). 

Golden White Cloud Minnow
Golden White Cloud Minnow

“Stocking” is how many fish of any given size can be put in any given size of aquarium. Stocking nano aquariums is simply a matter of OPINIONS and ONLY OPINIONS. The following are my OPINIONS

Stocking levels for a ten-gallon (40 liter) aquarium.

Total length of fish in centimetersLight stockingRecommended 100% level stockingHeavy stocking *
1in 2,5cm1863189
2in 5cm3928
3in 7,6cm139
4in 10c015
5in 12,7cm013
6in 15,3cm012
8in 20,3cm001
* ONLY with very heavy filtration
Number of fish for stocking in a 10 gallon or 37,85 liters aquarium
Oryzias woworae Daisy's Blue Ricefish
Oryzias woworae – Daisy’s Blue Ricefish

Stocking levels for a five-gallon (20 liter) aquarium.

Total length of fishLight stockingRecommended 100% level stockingHeavy stocking *
1in 2,5cm93190
2in 5cm1414
3in 7,6cm014
4in 10cm002
* ONLY with very heavy filtration
Number of fish for stocking in a 5 gallon or 18,93 liters aquarium

This stocking level is based on the fact that a four inch (10cm) fish has eight times the volume and the mass of a two inch (5 cm) fish. But studies have shown that a two inch fish has twice the metabolism of a four inch fish. So one can have four (eight divided by two) two inch fish and only one four inch fish in a five gallon. This relationship is often a little hard for beginners to grasp.

Trichopsis pumila, dwarf sparkling Gourami
Trichopsis pumila – Dwarf sparkling Gourami

With adequate filtration even a small aquarium CAN stock a lot of fish. Now whether or not it SHOULD stock a lot of fish is a matter of a huge amount of very passionate opinion and debate. The PETA folks think anything more than very light stocking is cruel and should be outlawed. But passionate opinion and scientific fact are not synonymous.

Sawbwa replendens Rummy Nose Rasbora
Sawbwa replendens – Rummy Nose Rasbora

The stocking levels above can and have been done quite successfully by MANY fishkeepers. The fish have NOT suffered needlessly. The fish have thrived.

The “recommended” stocking above is about 10% of the stocking level done by University research centers who breed millions of zebrafish for genetic research. The research demands that the fish stay healthy and that they “thrive”.
These researchers stock 30 one inch (2.5 cm) zebrafish in one gallon (4 liter) containers (but note they have very good aeration and filtration on these containers!).
This means the stocking for small aquariums is VERY flexible IF and ONLY IF there is adequate aeration and filtration.
Pigmy Rasbora
Pigmy Rasbora

Note that the heavy stocking levels (three times the “recommended” levels) are only viable with very good biofiltration and very good aeration. A cartridge hang on back or internal filter or a corner filter typically won’t meet the criterion of “good biofiltration”. Since these are typically the only filtration available for small aquariums the heavy stocking levels are crossed out.

Celestichthys margaritatus Celestial Pearl Danio
Celestichthys margaritatus – Celestial Pearl Danio

If one adds a small canister and/or a small undergravel filter and a medium sized air pump and air stones to a small aquarium one can go to the three times the “recommended” levels. Just don’t post a picture of it on social media. You will deluged with a whole host of very nasty comments.

And some ascribe to the mantra that fish need larger aquariums for “swimming room”. There is absolutely no scientific evidence of any such requirement. Fish do not need cardio workouts. There are some fish from fast moving streams such as trout which need to move in order to keep water flowing over their gills. I know of no aquarium fish that need that.

Fire Ring Danio
Fire Ring Danio


I think the best filter for any nano tank for fish is an under gravel filter. It looks good, works very well, doesn’t need ANY maintenance, and it is cheap. There is a lot of negative myths out there about under gravel filters. None of them are true. More about this can be found at this link:

8.5. Under Gravel Filters

There are some small sponge filters made for nano aquariums. Unfortunately a small five gallon (20 liter) aquarium is so small one cannot hide even a small sponge filter. There are also a whole host of small do-it-yourself filters that can be used in a nano aquarium. Some are especially useful in a planted nano tank. More about these can be found at this link:

8.7.8. Small DIY Filters

Elassoma evergladei Everglades Pygmy Sunfish
Elassoma evergladei – Everglades Pygmy Sunfish