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Related Articles: Freshwater Livestock by Neale Monks, Stocking a 10-gallon aquarium: some new ideas and old favourites for nano tanks by Neale Monks, The Ethical Aquarist; Freshwater Fishes to Avoid by Judy Helfrich Acclimation of New Freshwater Livestock, by Bob Fenner

Small tank options

A selection of species for freshwater nano aquaria



By Neale Monks 2015


If you're starting out in the hobby, the one thing to avoid doing is buying a tank that's too small. In fact the ever-popular 20-gallon tank really is an ideal size for beginners, being big enough to stock most of the popular species but compact enough to fit into most homes without problems. Whether you're keeping a pair of Angelfish or a school of Zebra Danios, a 20-gallon tank will work a treat!

But some aquarists like a challenge, and stocking very small tanks can be a very stiff challenge indeed. While a 10-gallon tank can be stocked safely if you avoid the bigger community species, tanks smaller than 10 gallons present some major problems. One problem is of course lack of space; while a singleton Neon might be wedged into a 5-gallon tank, a school of eight or ten specimens will need much more space than that, and keeping schooling fish like Neons in twos or threes simply won't work out in the long term. Oftentimes we buy our fish when they're still comparatively young, not always realising that something like a Platy or Corydoras catfish actually gets pretty big once mature, maybe two or three inches long, and needs not just swimming space but adequately robust filtration as well.

Nonetheless, there are some livestock options out there worth considering for nano freshwater aquaria. A few might even be considered good choices for beginners, such as the Siamese Fighting Fish, the Dwarf Mosquitofish and the Indian Ricefish. But for the most part these fish are best suited to more experienced aquarists who know how to maintain excellent water quality and can provided the right sort of water chemistry where required.


Minimum aquarium size in US gallons

Water chemistry requirements


Siamese fighting fish or Betta

Betta splendens

5 gallons

Farmed specimens are very adaptable, but avoid extremes

Air-powered filtration recommended; heater essential; must be kept on its own; jumps out of open-topped tanks!

Cherry Shrimp
Neocaridina davidi

5 gallons

Adaptable, but avoid very soft water

Probably the easiest shrimp to keep alongside nano tank fish species; sociable, so keep in groups; breeds readily; females are larger and bright red, males smaller and transparent pink; consumes algae, flakes and organic detritus; does well in unheated tanks in warm rooms; may jump out of open-topped tanks; quickly killed by copper-based medications; numerous other shrimps are available, varying in size and adaptability

Dwarf Mosquitofish

Heterandria formosa

6 gallons

Will not do well in soft or acidic conditions

US native that can be kept in unheated tanks in warm rooms; livebearer that breeds slowly but steadily; cannot be kept with other fish, but mixes well with small shrimps

Anchor Catfish
Hara jerdoni

6 gallons

Not fussy but avoid extremes

Comes from cool mountain streams, so avoid high temperatures (around 72 F is ideal) and ensure there's plenty of oxygen by not overstocking and keeping the filter running properly; nocturnal, very inactive; likes to dig into sand; eats tiny pellets and bits of flake as well as small frozen foods such as brine shrimps; not difficult to keep, and compatible with small, peaceful midwater species, but a fish for experienced hobbyists nonetheless

Scarlet Badis
Dario dario

6 gallons

Not fussy but avoid extremes

Territorial cichlid-like fish; provide plenty of shelters and keep just one male alongside a group of females; e.g., for a 6-gallon tank, one male and two females would work; for every two additional gallons, add another female; sexing easy, males being much more colourful (red with blue markings) than the females (which are greenish-brown with a few coloured spots); like all Badidae, these fish are extremely fussy feeders, taking live and frozen foods (daphnia and brine shrimp in particular) while ignoring dried foods; though not delicate, these are fish for moderately experienced aquarists only

Indian Ricefish
Oryzias melastigma

7 gallons

Medium hard to hard water essential; the addition of a little salt (1-2 tsp. per gallon) can be useful but isn't essential

Tropical fish that thrives at 77 F; keep in groups of 6+ specimens; easy to breed, the females laying their large, sticky eggs on floating plants; the large fry can be reared on powdered flake food

Oryzias latipes

7 gallons

Medium hard to hard water essential; the addition of a little salt (1-2 tsp. per gallon) can be useful but isn't essential

Schooling fish that may be kept in unheated tanks in warm rooms; care similar to Indian Ricefish as described above; an easy-to-keep favourite among scientists, a genetically-modified fluorescent form is sometimes offered for sale in the US (though these shouldn't be confused with fluorescent Danios, known as GloFish)

Daisy's Ricefish
Oryzias woworae

7 gallons

Adaptable, but avoid very soft water

Care similar to the Indian Ricefish; this species is exceptionally colourful, and one of the most popular species for nano aquaria in the UK

Micro Rasbora
Boraras micros

7 gallons

Soft water only; 1-12 dH, pH 6-7 recommended

Boraras live in large schools in the wild, so keep 10+ specimens; rather delicate and sensitive, so recommended for expert fishkeepers only

Mosquito Rasbora
Boraras brigittae

8 gallons

Not an option for hard water; 1-12 dH, pH 6-7 recommended

Probably the easiest of the Boraras dwarf rasboras to keep; shy, so keep 10 or more specimens; dominant male develops especially strong colours; prefers tiny live foods, but will eat finely powdered flake as well

Ember Tetra
Hyphessobrycon amandae

8 gallons

Avoid extremes; ideally kept in soft water, but medium-hard water okay; 1-15 dH, pH 6-7.5

Peaceful and easy to keep in soft water tanks; keep 8+ specimens; can look washed out in Spartan tanks, but large groups in shady, well-planted systems colour up nicely

Galaxy Rasbora
Danio margaritatus

8 gallons

Adaptable, but slightly hard water around neutral in pH works well; 5-15 dH, pH 6.5-7.5

Exceptionally attractive fish when kept properly; groups of at least 6 specimens necessary to avoid problems with aggression between males; not too warm water (around 75 F) with plenty of oxygen essential to long-term success; eats dried foods happily enough but live or frozen foods are well worth adding to the menu; completely peaceful, best kept on its own or alongside quiet bottom dwellers such as Cherry Shrimps or Anchor Catfish


A note about stocking density

Apart from the Siamese Fighting Fish, Scarlet Badis and the Anchor Catfish, the fish listed here are all gregarious species and should be kept in groups. Because they're all very small species, the "inch per gallon" rules works very well for them. It's a good idea to err on the side of caution though with the very small tanks, those measuring six gallons or less in capacity. In other words, while 6-gallon tank could easily play home to half a dozen Ricefish, an 8-gallon one might be used to house 8-10 such fish.

A further note, this time on mixing species

Unless otherwise noted, none of these fish are "community fish" in the sense of being easily combined with other species. In fact nano tanks are best stocked with just the one species. Because these fish are so small, they're at the bottom of the food chain in their natural habitats, so keeping as many as you can will ensure they feel secure. That in turn will encourage them to swim about in the open and display their best colours.

Cherry Shrimps are an exception to this rule. They work well with most nano fish species (though not Siamese Fighting Fish, which tend to view them as food) and given enough shade and shelter may even multiply when kept in nano tanks.


Some of the fish listed above are routinely traded; others will require some effort tracking down. But all make much better choices for nano tanks than "small" community fish like Neons, Guppies and Danios, and it cannot be stressed too strongly that any aquarium smaller than 10 US gallons can't really be used for casual fishkeeping without a great deal of forethought and research. On the plus side, there are certainly worthwhile options for unheated tanks, for soft water tanks, and for hard water tanks; just because you're limited to a 6- or 8-gallon aquarium doesn't mean you can't keep something interesting, attractive or breedable!


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