49 Fun Facts About Aquarium Fish

The world's first public aquarium opened in 1853 at which zoo?

London Zoo's Fish House was stocked by the naturalist Philip Henry Gosse, whose 1854 book The Aquarium gave the word its modern meaning and set off a Victorian craze for keeping tanks at home.

The bala shark sold for home aquariums is no shark at all but a relative of which fish?

Carp are its kin: the bala shark owes its name only to a torpedo-shaped body and tall fins, and as a shoaling fish that reaches 35 cm it soon outgrows most home tanks.

Some of the earliest known fishkeepers were which ancient people, around 2500 BCE?

Sumerians kept fish in artificial ponds at least 4,500 years ago; the Romans later did the same with sea fish, in ponds supplied with fresh seawater.

The goldfish was first bred for colour more than 1,000 years ago in which country?

China's breeders started from a dull, olive-grey pond fish with the odd golden mutant; let goldfish loose in a river and within generations they revert to that wild colour and can reach 30 cm.

A popular myth, long since disproved, says a goldfish's memory lasts only how long?

Three seconds is the myth; in tests goldfish learn which paddle earns food, escape nets and run mazes, and can still repeat the tricks weeks or even months later.

Which venomous aquarium fish from the Indo-Pacific has overrun Caribbean reefs, probably after owners dumped their pets?

Lionfish have very few predators in the Atlantic and eat the young of snapper and grouper, so divers in Florida and Texas are encouraged to spear them for sport and for the table.

The Bubble Eye goldfish is distinguished by fluid-filled sacs under its eyes and the lack of which fin?

Dorsal fin missing, the Bubble Eye struggles to keep its balance, and its sacs are easily sucked against a filter intake; a damaged sac usually grows back, though not always the same size.

Which aquarium fish knocks insects off overhanging leaves with a jet of water spat from its mouth?

Archerfish aim from below the surface, so they have to allow for the way light bends between water and air, and they can still hit an insect more than a metre above them.

Which organ lets bettas gulp air and survive poorly oxygenated water?

The labyrinth is a chamber of folded, bony plates above the gills that takes oxygen from gulped air; many of these fish rely on it so heavily that they must reach the surface to survive.

What do male bettas build at the surface of the water to hold their eggs?

A bubble nest keeps the eggs at the surface, where oxygen is richest; the male gathers each sinking egg, lodges it among the bubbles and guards the raft until the fry hatch.

The guppy is named after Robert John Lechmere Guppy, who sent specimens to London from which island?

Trinidad was home to Robert Guppy, an engineer and schools inspector rather than a career zoologist; the fish already had a scientific name from Venezuela, so his surname survived only as its everyday name.

Guppies, mollies, platies and swordtails are all what kind of fish?

Livebearers give birth to free-swimming young instead of laying eggs, and a female can store sperm for months, so a single mating can yield a new brood roughly every four weeks.

What is the gonopodium of a male guppy or platy?

A modified anal fin used for mating, the gonopodium is rolled into a tube that passes bundles of sperm to the female; her own anal fin stays fan-shaped, the quickest way to tell the sexes apart.

The 'sword' of the swordtail, a long spike on the lower edge of the tail fin, is grown by which fish?

Adult males only, because females choose mates with longer swords, even though a long sword also makes its owner easier for predators to spot; the female is the bigger fish but has none.

The platy crosses so readily with which close relative that most shop fish are said to be hybrids of the two?

Swordtail and platy belong to the same genus and cross so freely that few aquarium strains are pure, which also made the pair standard subjects for genetics research.

The neon tetra, one of the most widely kept tropical fish, is native to which river basin?

The Amazon's acidic forest streams are its home, yet few shop neons have ever seen them: the great majority on sale are bred on commercial farms rather than caught.

How can you tell a cardinal tetra from a neon tetra?

Its red runs the whole body length, where a neon's stops about halfway; and while shop neons are farm-bred, many cardinals on sale were still caught wild in tributaries of the Rio Negro.

Which tall, flat-sided cichlid with sail-like fins and dark vertical bars is a mainstay of tropical aquariums?

Angelfish pair up and spawn on a vertical surface such as a broad leaf, or the aquarium glass or filter pipe if nothing better is on offer, then stand guard over the eggs and fry.

Discus fish feed their newly hatched fry on what?

Mucus secreted from their own skin is the fry's first food: they graze it straight off their parents' sides while both adults stand guard over the brood.

Clownfish live unharmed among the stinging tentacles of which animal?

Sea anemone stings that would kill other small fish are blunted by a protective coating the clownfish wears; in return the fish drives off intruders and keeps its host clean.

Clownfish are protandrous hermaphrodites, meaning what?

A male can change into a female, never the reverse: every clownfish starts out male, and only when a group's dominant female is lost does her mate switch sex to take her place.

Dory in Finding Nemo is which species, a favourite of marine aquarists?

Regal blue tang in shops are still mostly taken from wild reefs rather than bred; cornered, the fish raises a razor-sharp, venomous spine on either side of its tail.

Which large South American cichlid, also called the velvet or marble cichlid, is eaten as food in its homeland?

Oscar is esteemed on the table across tropical South America and sold in local markets, yet it grows too slowly to be worth farming, so it is the aquarium trade that made its name.

What protects Corydoras catfish along each flank in place of scales?

Overlapping armour plates of bone run in two rows down each side; corys also dash to the surface to swallow air, which they absorb through the gut, so they cope in water short of oxygen.

The common pleco is bought by hobbyists mainly for what job?

Cleaning algae as a bottom cleaner comes naturally to a suckermouth catfish: the mouth clamps on to glass, rock or wood and rasps off whatever is growing there.

Bristlenose or bushynose plecos belong to which genus?

Ancistrus comes from the Greek for 'hook', after the males' curved cheek spines; it is also the males that grow the fleshy tentacles on the head, and that guard the eggs in a cave.

GloFish, the fluorescent pets that reached American shops in 2003, were first engineered from which common aquarium fish?

Zebra danio made to glow were among the first genetically engineered animals sold to the public; the FDA waved them through in December 2003, reasoning that aquarium fish are not eaten.

Koi breeding for colour began in the early 19th century in the hill villages of which Japanese prefecture?

Niigata's hill farmers kept plain black fish in ponds as winter food; when the odd coloured mutant turned up they prized it and bred from it, and a century later the results were being called 'brocaded' koi.

Hanako, a scarlet koi that died in Japan in 1977, was claimed from her scale rings to have reached what age?

226 years is the age claimed for her when she died, from rings counted on her scales like those of a tree; the count was never published, and an ichthyologist asked about it said scales are the least accurate way to age such a fish.

Many African lake cichlids protect their eggs and fry by carrying them where?

In the mother's mouth the eggs hatch and the fry stay for up to three or four weeks; she barely eats in that time, and a brooding female can be spotted by her bulging jaw.

Why can many killifish eggs survive partial drying out?

They live in pools that dry up each year: adults spawn into the mud and die, and the embryos lie dormant in it until the rains return, a life so brief that these fish are used to study ageing.

The word 'killifish' most likely comes from a Dutch word meaning what?

Small stream is what 'kill' meant in the Dutch of the early American colonies, which is why the map there still shows the Catskills, the Schuylkill and Fresh Kills; the fish is probably just a 'kill fish'.

Which Asian aquarium fish have pelvic fins drawn out into long, thread-like feelers?

Gouramis carry one long, feeler-like ray at the front of each pelvic fin and reach out with it to touch things; a courting pair can be seen touching one another with them before they spawn.

What marking identifies the harlequin rasbora?

A black flank triangle is its badge, a dark wedge tapering back to the tail; and where most of its small relatives scatter their eggs, it sticks them to the underside of a broad leaf.

Brackish aquariums, home to monos and scats, hold what kind of water?

Part fresh, part sea water is what estuaries and mangroves hold, and several fish sold as freshwater species, mollies among them, also inhabit it in the wild.

In the aquarium nitrogen cycle, bacteria convert toxic ammonia first into what?

Nitrite, itself highly toxic, is turned into far milder nitrate by a second group of bacteria; a new filter needs weeks to grow enough of both, which is why a brand-new tank is risky for fish.

In a reef aquarium, what is a refugium?

A side tank for growing helpful algae works as a living filter: the seaweed soaks up nitrate and phosphate, and tiny crustaceans can breed there out of reach of hungry fish.

The tiger barb is native to which Indonesian island?

Sumatra is its home, hence its other name, the Sumatra barb; kept in ones and twos it turns on its tank-mates' fins, so keepers are told to buy a shoal and let the barbs squabble among themselves.

Which eel-shaped loach burrows into sand for long spells?

Kuhli loach takes its name from Heinrich Kuhl, a German naturalist; under each eye the fish hides a sharp spine that springs up when it is stressed and can snag in a net.

The popular red cherry shrimp is a colour morph of a species whose wild form is what colour?

Mottled brown is the shrimp's natural camouflage, and the bred colours do not hold by themselves: let red, blue and yellow strains mix in one tank and within a few generations the young drift back to it.

Which aquascaper, who popularised the 'nature aquarium' style, is the namesake of a popular algae-eating shrimp?

Takashi Amano put the shrimp to work clearing algae from his planted layouts in the 1980s, and the trade named it for him; its newly hatched larvae drift downstream to salt water to grow.

Which small aquarium fish was first collected by a Chinese Boy Scout leader named Tan, remembered in its genus Tanichthys?

White Cloud minnow was feared extinct in the wild after two decades without a sighting, until small populations turned up again from 2001; albonubes, its species name, points to the mountain near Guangzhou where Tan found it.

The German blue ram comes from the grassy plains of Venezuela and Colombia in which river basin?

The Orinoco's grassy plains are its wild home; the 'ram' is short for Manuel Ramirez, an early collector of the fish for the aquarium trade, and it does best in very warm water, in the high twenties Celsius.

Rainbowfishes of the family Melanotaeniidae come mainly from which part of the world?

Australia and New Guinea, with a few nearby islands, hold most of them; the colour is mostly the males' doing, and they show it best in a shoal, in the company of their own kind.

Pet goldfish dumped in lakes across North America and Australia spoil the habitat for native wildlife in which way?

Clouding the water with mud is what they do as they root along the bottom for food: less sunlight reaches the underwater plants, and the dumped pets themselves can grow to the size of a football.

A male betta shown a mirror will usually do what, taking its reflection for a rival?

Flare its gills and fins is the betta's standard threat: the gill covers swing out and every fin spreads to make the fish look bigger, which is why two males cannot share a tank.

The guppy is also known by which nickname, a nod to how fast it breeds?

Millionfish were shipped around the world to eat mosquito larvae; they did little to the mosquitoes but settled in far beyond South America, often at the expense of native fish.

Bettas were first bred in which country, where fights between males drew gamblers?

Thailand's breeders first selected males for aggression and only later for the colours and flowing fins that made the betta a pet; it has since been named the country's national aquatic animal.

Which large South American aquarium fish, often dumped when it outgrows its tank, has teeth that look strangely human?

Pacu use those blunt, square teeth to crack nuts and seeds; released pets have turned up in lakes as far from home as Michigan, where anglers have hooked them.