8 Best Fish for Small Tanks in Australia

A betta is the one fish on this entire list that is actually built for 20 litres, not just tolerant of it. Every other entry here wants a group, and a group needs litres a 20-litre tank does not have. A betta needs none of that: one fish, alone, in a real filtered and heated tank, not the cup or bowl it is still sometimes sold from.

Whirlpool’s own aquarium forum has multiple Australian keepers reporting a single male betta thriving in exactly this bracket, paired with two albino corydoras and a school of neon tetras in a 40-litre setup, though at 20 litres flat I would leave it solo. Skip the mirror trick some shops suggest for “showing off” the fins. It just stresses the fish for a few seconds of flaring, and it does nothing for a betta kept correctly.

Compared to every schooling fish below, a betta is the only entry that does not scale its need for space by whether you add three fish or eight. That is the entire pitch and the entire limitation in one line.

If the betta above is the solo answer to a 20-litre tank, the chili rasbora is the schooling answer. At 2 centimetres fully grown, a group of eight fits a genuine 20-litre footprint the way six neon tetras never will, and a proper group is what turns this fish from “a red speck” into the shimmering red shoal it is kept for.

The trade-off is price and patience. Chili rasboras run noticeably more per fish than an ember tetra or a guppy, largely because they are still wild-caught or limited-scale tank-bred instead of mass-produced, and Australian specialty retailers price them accordingly. They are also shy new arrivals. Give them dense planting and floating cover for the first fortnight and they settle; drop eight of them into a bare tank and half will hide behind the filter intake.

Where chili rasboras beat the celestial pearl danio just below is scale. Both are true nano fish, but the danio’s more active swimming pattern wants a touch more floor space than a chili rasbora school does.

Also sold as the galaxy rasbora, and that older name is still what a lot of Australian shops and forum threads use. A 2016 Whirlpool thread on stocking a 40-litre tank has an experienced keeper recommending galaxy rasboras over zebra danios outright, calling them the nicer-looking fish, and that recommendation has aged well. They are noticeably more attractive up close than a plain schooling tetra, with a scattered pearl pattern over a dark body and orange-red fins on the males.

They run cooler than most of this list. Twenty to 24 degrees C is comfortably below what a heater set for a standard tropical community tank delivers, so check your heater’s actual setting instead of assuming “tropical” covers it. In a lot of Australian homes through spring and autumn they will happily go unheated for stretches, though I would not rely on that through a Melbourne or Hobart winter.

Set against the ember tetra further down, celestial pearl danios cost more and want a touch more room, but the colour and the more natural, foraging behaviour in a planted tank is the reason keepers pay the difference.

Embers are the cheapest true nano schooler on this list, and the same 2016 Whirlpool thread flagged them as “very hardy” alongside the standard silvertip and cardinal tetra recommendations, which tracks with how widely and cheaply Australian retailers move them; several run six-for-twenty specials that work out under four dollars a fish.

At full size they barely clear 2 centimetres, smaller than a neon tetra, which is exactly why they scale down to a 30-litre setup that a neon school would find cramped. Colour intensifies with a darker substrate and won’t show at all under harsh white light, so if your tank looks washed out, that is the tank, not a dud batch of fish.

Where embers lose to the celestial pearl danio above is visual interest per fish. Embers are a solid orange-red block of colour and not much else, while danios carry an actual pattern. What embers win back is price: you can stock double the group for the same budget.

The standard bronze corydoras that turns up on every beginner list needs 60 litres for a proper group. This one does not. Pygmy corydoras top out around 2.5 centimetres, spend more time hovering mid-water than digging like their bigger cousins, and settle into a 30-litre setup with a group of six without crowding it.

They carry the same substrate warning every corydoras does, and it is worth repeating here since a small planted nano tank is exactly where keepers reach for a decorative sharp gravel: their barbels, the whiskers around the mouth, wear down on rough substrate over months. Sand is the safer floor for this fish, not a stylistic preference.

Compared to the otocinclus lower down this list, pygmy corydoras are the easier nano bottom-dweller for a newly cycled tank. Otocinclus need an established tank with real algae to graze; pygmy corydoras will eat a proper sinking pellet from day one.

Endler’s are basically small, colourful guppies. Not quite, though close enough that most keepers assume they behave the same way, and mostly they do: livebearers, breed readily, tolerant of a new tank’s rough edges. Where they earn a separate spot on this list is size. A male tops out around 2.5 centimetres, noticeably smaller than a fancy guppy, and a small group of five settles into 40 litres in a way a guppy group starts to outgrow.

One real caveat, and it is not a small one: endler’s and guppies will crossbreed, and the resulting fry lose the purebred endler’s pattern within a couple of generations. If you want to keep the strain clean, do not mix them with any guppy, ever, including one a friend swears is “probably fine.”

Set beside the betta above, endler’s give you constant group activity and colour instead of one fish’s individual display. Which one suits you is really a question of whether you want to watch a shoal move or a single fish hold its space.

This is the one genuinely quiet, easy-to-overlook fish on this list, and it rewards a keeper who wants something other than another schooling shoal. Sparkling gouramis are labyrinth fish, meaning they gulp air at the surface as well as breathing through their gills, and they make an audible croaking sound during courtship, a strange, small detail most first-time keepers have never heard of until they own one.

They are shy rather than fragile. A dense floating plant cover and calm tankmates matter more to this fish than exact water numbers do, and a lid is not optional. Labyrinth fish surface for air in a warm, humid layer just above the waterline, and a gap under an ill-fitting lid can dry that layer out on a hot day.

Compared to the betta, another labyrinth fish on this list, sparkling gouramis are the pick if you want a small group instead of a solo display, though neither fish tolerates fast, boisterous tankmates well.

I am putting this one last for a reason, not because it is a weak fish but because it is the one entry here with a real hype-versus-reality gap. Otocinclus are marketed constantly as a drop-in algae solution for a new tank, and that is close to the opposite of correct. The single most common cause of death in home-kept otocinclus is starvation in the first month, arriving thin from the wholesaler and going into a freshly cycled tank with no algae film built up yet to graze on.

Add them to an established, several-months-old tank with real algae growth on the glass and decor, not a tank you just finished cycling last week, and that risk mostly disappears. Keep a group of three or more; a lone otocinclus does poorly, and this is not a fish where “just one for now” is a safe shortcut.

Set against the pygmy corydoras above, otocinclus are the pickier, more specialist choice: better at actual algae control, worse at tolerating a young tank, and worth the wait rather than the first fish you add.