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Fish questions kids and parents ask

These are the fish questions people actually type into Google. Each answer is short. Where we have written a full guide, the answer links to it. — Yechiel Kuperman, marine biologist

Can one guppy live alone?

A guppy can survive alone, but it lives better with company. Guppies are shoaling fish: in the streams of Trinidad they swim in loose groups, and a single guppy in a tank usually hides more and shows less of its natural behaviour, like the males' courtship displays. For a home tank, keep a small group of male guppies. Males only is the simple beginner rule, because females can have a new brood about once a month and a mixed tank fills up fast. Give the group a heated, filtered tank, never a bowl. If you want to meet a guppy with strong opinions about his water, start with George the Cranky Guppy, Book 1.

How many guppies should you keep together?

Keep at least three, and five or six is better if the tank has room. A group spreads out, swims more and shows the colour and display that make guppies fun to watch. For beginners, choose males only: they are the colourful ones, and with no females there are no surprise babies every month. Add them once the tank has finished cycling, and do not add more fish than the filter can handle. Our guide to how many fish fit in a tank explains why the old "one inch per gallon" rule is wrong. And if your child likes guppies, they may like George, a guppy with opinions.

How long are guppies pregnant?

Usually about three to four weeks in a warm tank, and longer in cooler water; FishBase gives four to six weeks across conditions. Guppies are livebearers, which means the babies, called fry, grow inside the mother and are born already swimming. FishBase lists about 20 to 40 young per birth. A pregnant female grows rounder, and the dark spot near the back of her belly gets bigger and darker. A female can store sperm and have several broods from one mating, so a female from the shop may give birth even in a tank with no males. Thick plants give the fry places to hide. Want more guppy science for kids? It is woven into the George the Cranky Guppy books.

Do guppies need a heater?

In most homes, yes. Guppies come from warm tropical streams and do best in steady water of about 24 to 26 °C (75 to 79 °F). Most living rooms are cooler than that, especially at night and in winter, and cold water slows a guppy down and makes illness more likely. Use a small adjustable heater and a separate thermometer placed away from the heater, so you read the real water temperature. Change the temperature slowly, by no more than about 1 °C a day. Steady matters more than the exact number. Our aquarium temperature guide has heater sizes. For the story side of fish and water, see the George books.

Can guppies live with bettas?

It is best not to try. A male betta is a territorial fish, and the long, bright tails of male guppies can look like a rival to him, so he may chase and nip them. Guppies can also nip a betta's long, slow fins. Some keepers make it work in a large, planted tank, but for a child's first tank the simple rule is better: a single male betta in his own heated tank of at least 19 litres (5 US gallons), or a small group of male guppies in another tank. Our guide to the best first fish for a child compares the options. For guppy stories with real science, see the books.

What is the most friendly pet fish?

No fish wants to be cuddled, but some clearly notice you. A single male betta often swims to the front of the glass when its owner walks up, and it soon learns feeding time. A group of male guppies or platies is busy and colourful, and they gather at the front of the tank when they see you coming. Goldfish learn people too, but they grow far bigger than most children expect and need a large tank. The best "friendly" fish for a child is the one whose tank you can keep clean, because a healthy fish is an active, curious fish. New to fish and to our books? Start here.

Why is my fish tank cloudy?

In a new tank, milky white water is usually a bloom of bacteria while the tank is settling in. It tends to clear by itself as the filter matures, so do not keep changing all the water or adding chemicals. In an older tank, cloudy water usually means too much food: uneaten flakes rot and feed the bacteria. Feed less, take out what is not eaten within a couple of minutes, and test the water for ammonia. Remember that clear water is not the same as clean water, and only a test kit tells you which you have. Our guide to waste and ammonia explains the tests. Water that feels wrong is where George's story starts.

Why is my fish tank water green?

Green water is algae, tiny plants floating in the water. Algae need two things: light and nutrients. Too many hours of light, or sunlight from a window, plus waste from extra food, makes them bloom. Most fish are not harmed by the green itself, but it tells you the tank has more light and waste than it needs. Keep the light on for eight to ten hours a day with a plug-in timer, move the tank out of direct sun, feed a little less and keep up regular partial water changes. Live plants help too, because they use the same nutrients. More fish science, told as a funny story, is in the George the Cranky Guppy books.

Why is my fish swimming upside down?

A fish swimming upside down is in trouble, not playing. Healthy fish stay upright, even when they rest. Losing balance usually means the swim bladder, the gas-filled organ a fish uses to float, is not working properly. That most often follows another problem, such as poor water, constipation or an infection. Test the water first, because bad water is the most common cause and the easiest to fix. Check the temperature, feed lightly and watch the fish closely. If a child spots it, they should tell a grown-up straight away. Our guide to fish warning signs explains each sign. For a fish story where a grown-up notices what is wrong, read the books.

Why is my fish not eating?

A new fish often skips food for a day or two while it settles in, which is normal. In a fish you have kept for a while, refusing food is one of the first signs that something is wrong, usually the water. Test for ammonia and nitrite, check the temperature, and look for other signs: clamped fins, hiding, or gasping at the surface. Do not keep adding food, because uneaten food rots and makes the water worse. Healthy adult fish can go one to three days without food with no harm, so there is time to find the cause. Our guide to how much to feed fish has the details. A fish who notices his water is wrong is George, in Book 1.

Answered in our guides

More numbers? See our pet fish statistics. A question we have not answered? Ask us.