Choosing a goldfish and bringing it home

By the time a goldfish is swimming in your tank, most of what will determine how it does has already happened. Which shop it came from, how long it had been there, what the journey was like and what you did in the first hour matter more than anything you will do in the following week.

None of it is difficult. Almost all of it is free. And a very useful item on the list is a question you ask before you buy.

Before you go

Have the tank running first. Cycled, or at least started, with the filter established. The cycling guide explains why this is the one preparation that cannot be improvised afterwards, and buying the fish first is a common sequence and the wrong one.

Know what you are buying. Fancy or single-tail is the decision that shapes everything downstream — tank size, temperature range, food, tankmates — and it is far easier to make at home than under shop lighting with a child beside you. The breeds section covers each one; the short version is that fancies suit an indoor aquarium and single-tails want a pond or a very large tank.

Know how many. Not how many the tank will eventually hold, but how many to add now, which is one or two. The stocking checker does the arithmetic for the tank as a whole.

Have the quarantine tank seeded, if you are going to use one. Its sponge filter should have been running in the main tank for weeks, as the quarantine guide sets out — which is the argument for reading this page a month before you shop rather than on the day.

Read the shop, not the tank

This is the part most people skip and it carries more information than anything else.

Goldfish in a shop are usually on a shared water system — a bank of tanks plumbed into one sump and one filter. That means the water your fish is swimming in has also passed through every other tank on that run, including any containing a sick fish.

So look along the whole system:

• Any dead fish, in any tank on the run. One is a reason to leave. Fish die in shops and a good shop removes them promptly; a body still in a display tank tells you how often anyone looks.
• Visible disease anywhere — white spots, fungus, ragged fins, fish hanging at the surface. Again, on any tank sharing water.
• Cloudy water or a smell, which suggests the system is loaded beyond what it can handle.
• Crowding. Some density is normal in a retail system. Fish with no room to turn is not.
• Staff who know the answers. A shop that can tell you when a batch arrived and what it is fed is a shop that is paying attention.

A single sick fish in an otherwise excellent shop is bad luck. A pattern is information.

Read the individual fish

Then look at the one you want, using the same checks as the healthy baseline page.

Look at	Want to see
Fins	Held open and moving, not clamped against the body
Breathing	Steady and unhurried, both gill covers together
Eyes	Clear, matching each other, allowing for the breed
Body from above	Symmetrical, no lumps, scales lying flat
Skin and fins	No spots, tufts, red streaks, missing scales or open sores
Swimming	Level, controlled, using the tank
Behaviour	Alert, interested, not hiding or hanging in a corner

And ask to see it fed, if the shop will. A goldfish that eats enthusiastically in front of you has told you more than any amount of looking.

What to refuse

Any fish from a tank containing a dead or obviously sick one, however good the individual looks.

A fish that arrived this week. Stock that was caught, bagged, transported and released within the last few days is still stressed and has not had time to show whether it is carrying anything. Ask when the batch came in; a week settled in the shop is worth waiting for.

A fish that is hiding, hanging, clamped or breathing hard, even if everything else in the tank looks well.

A fish sold as something it is not. “They only grow to the size of the tank” and “it will be fine in a bowl” are both still said, and a shop that says them is not a shop whose stock advice you should trust.

A fish you are buying to rescue it. This is the hardest one, because the impulse is decent. But a fish bought out of pity from a bad tank goes into your tank carrying whatever was in that system, and it funds the practice. If a shop’s conditions are poor, the useful responses are to leave and to say why.

Questions worth asking

Four, and they take a minute.

“When did these arrive?” A very useful question on this page.

“Are these tanks on one system?” Determines how far your inspection needs to extend.

“What temperature do you keep them at?” So you can match it, and so you know whether they have been kept tropical — which some general pet shops do, and which means a larger adjustment than you might expect.

“What are they fed?” Useful for the first week, since a fish that is already eating a familiar food is one less change.

The bag and the journey

Ask for a reasonable air space. The bag should be roughly one third water and two thirds air, which gives the fish a surface to draw oxygen from. A bag filled to the top with water looks generous and is worse.

Ask for oxygen if the journey is more than an hour. Most aquatic specialists will do it; general pet shops often cannot.

Keep it dark. A paper bag, a box, or simply the footwell rather than the back seat. A fish that cannot see movement is a fish that is not panicking, and shops bag fish into opaque paper for exactly this reason.

Keep it stable and go straight home. No boot in July, no boot in January, no further errands. The journey is the most stressful part of the whole process and shortening it is the main thing you control. Handling, netting and moving a goldfish covers longer journeys and every other move a fish will make after this one.

The first hour

Once home, the sequence is short and the order matters.

1. Float the sealed bag in the tank for about fifteen to thirty minutes, with the tank light off. This brings the water in the bag to tank temperature, the main thing to match for a goldfish.
2. Dim the lights, or turn the tank light off entirely.
3. Open the bag and add tank water in stages — a cupful every few minutes for up to about twenty minutes, less after a long journey, when the bag holds more ammonia — so the fish meets your water chemistry gradually rather than instantly.
4. Net the fish out and into the tank. Do not pour.
5. Pour the bag water down the sink.
6. Leave the lights off for the rest of the day and do not feed until the next morning.

If the shop’s water and yours differ by more than about 10 °C, spread the change over two hours or more, adding tank water to the bag in small, frequent amounts.

The first hour home

1. 1 Float the sealed bag About 15–30 minutes with the tank light off, so the water in the bag reaches tank temperature.
2. 2 Add tank water in stages A cupful every few minutes for about twenty minutes, so the fish meets your water gradually.
3. 3 Net the fish across Into the tank, by net. Do not pour the bag in.
4. 4 Bag water down the sink It has been collecting ammonia all journey. Lights stay off, and no food until the morning.

Longer acclimation is not better. The instinct to extend the process — a slow drip over an hour, a bag floated all afternoon — comes from marine and invertebrate keeping, where chemistry differences are severe. A goldfish is a hardy freshwater fish whose main requirement is temperature, and the bag is deteriorating the whole time you wait.

The first forty-eight hours

No food on day one. A fish that has spent the day in a bag has enough to recover from, and uneaten food in a tank with a new bioload is an unnecessary risk.

Lights low or off. Reduce what the fish has to react to.

No introductions. Do not stand at the glass, do not let children tap, do not rearrange anything.

Watch, without interfering. The things worth looking at are breathing rate, fin position and whether the fish is exploring or sitting. A new arrival that hides for a day is normal; one that is still hiding and clamped on day three is not.

Feed lightly on day two, and see whether it eats. Appetite returning is the clearest sign the move went well.

Then quarantine

If the fish is joining an established tank, everything above is the prologue to four weeks or more of isolation, which is the subject of its own guide. If it is going into a tank of its own with nothing to infect, the isolation is unnecessary — but the close watching is not.

The size you are shown is not the size you are buying

This is a very consequential thing about buying a goldfish, and the shop will not usually raise it.

Adult size depends mainly on the breed, and the first thing you can check through the glass is whether the tail is single or double, because single-tail strains grow larger than most fancies. What kind of goldfish do I have? is the two-question key, and this is the moment it is worth most — a single-tail bought for a small tank is a problem that can take a year or more to appear.

Goldfish are sold small. A typical shop fish is three to six centimetres, which looks entirely at home in a twenty-litre tank on a shelf. Fully grown, which takes years, a fancy is usually fifteen to twenty centimetres and a single-tail twenty-five to thirty-five, and neither will fit the tank it was sold alongside.

At purchase	Adult size	First-fish volume it will need
Fancy, 3–5 cm	15–20 cm	75–100 L
Single-tail, 4–6 cm	25–35 cm	150–200 L, then a pond

The mismatch is not a trick anyone consciously plays. It is simply that fish are sold at the size they are economical to produce and transport, and that “how big will this get?” is a question few buyers think to ask about an animal in a small tank.

Buy for the adult. The tank size guide sets out the figures, the growth page covers why a small tank stunts the body rather than politely limiting it, and the real consequence is that a goldfish is not a small-tank animal at any price.

Price, and what a very cheap fish means

Goldfish are inexpensive, and that is part of the problem — an animal that costs less than a coffee is an animal people are willing to replace rather than treat.

Within the market there is a genuine spread, and it carries information. Feeder-grade goldfish, sold in bulk at the lowest price, are bred for volume rather than health, kept at very high densities, and are the stock most likely to arrive carrying something. Ordinary pet-shop goldfish occupy the middle. Specialist-bred fancies cost several times as much and are usually better fish: better conditioned, better shaped, and from systems with far fewer animals in them.

None of this means the cheapest fish is doomed or the dearest is guaranteed. It means price is one more signal alongside the shop’s condition and the fish’s own appearance, and that a bargain is worth a second look rather than a first grab.

The running costs are the same whichever you buy. A goldfish that costs two pounds still needs the hundred-litre tank, the filter, the test kit and the decade of water changes, which is the real budget and the subject of a conversation worth having before rather than after.

Sex, and why you cannot pick

People often want a male and a female, and outside breeding condition you almost certainly cannot tell which is which.

Goldfish show no reliable external difference most of the year. In condition a male develops small white breeding tubercles on the gill covers and pectoral fin rays and a female fills out through the belly, but shop fish are usually young, usually out of season and usually not displaying either.

This matters less than it seems. Goldfish do not pair-bond and do not need a mixed pair, and a deliberately mixed pair in a tank that suits two fish becomes a tank that does not suit the outcome. If you get a spawn, the breeding page covers the arithmetic — and the summary is that it is not a small event.

If it dies in the first week

It happens, it is not always anybody’s fault, and there are two separate questions.

The practical one. Test your water immediately, before changing anything, because a death in the first week is as likely to be an uncycled tank as a sick fish, and the reading distinguishes them. The why a goldfish dies page works through what the water is telling you.

The commercial one. Many aquatic shops offer a short guarantee, typically a few days to a fortnight, usually conditional on bringing in a water sample. That condition is reasonable — it separates a fish that was already unwell from a tank that was not ready — and a shop that tests your water and talks you through it is doing you more good than a refund.

What is worth resisting is the instinct to immediately replace the fish. If the tank killed one, it will kill the next. Find out which of the two it was first.

A checklist to take with you

Three moments, and what has to be true at each. Nothing here is optional, and the order matters: the first group has to be finished days before the other two are worth starting.

Before leaving home

• The tank is cycled, with ammonia and nitrite reading zero.
• The temperature is within range and steady, not still climbing.
• The quarantine tank is seeded and running, if you are using one.
• You have decided fancy or single-tail, and how many you are buying today.

At the shop

• No dead or visibly sick fish anywhere on the system, not just in the one tank.
• The batch arrived a week or more ago. Ask; a fish that landed yesterday is still in transit as far as its immune system is concerned.
• Fins open, breathing steady, eyes clear, body symmetrical.
• You have watched it eat, if the shop will feed for you.
• The bag is one-third water and two-thirds air, with oxygen for a long trip.

Getting home

• The bag stays dark and stable, and you go straight home.
• Float the sealed bag 15–20 minutes, then mix your water in over another 20.
• Net the fish across; the bag water goes down the sink, not into the tank.
• Lights off, and no food until tomorrow.

Fairground fish, and other unplanned arrivals

A goldfish arrives without any of this preparation more often than people admit: won at a fair or a carnival, inherited from a house move, handed over by someone who has given up.

The practical response is the same in each case and the tone is worth saying explicitly — this site’s position is that the conditions are the problem, not the person holding the bag.

Get it into stable water. If there is no tank, a clean food-grade container with dechlorinated tap water at roughly the right temperature, plus an air stone if you can get one, is far better than a bag or a bowl.

Treat it as unquarantined. It has come from an unknown system and, in the fairground case, from a bag that has been warm for hours.

Know what clock you are on. A show bag holds perhaps a litre and has been warm and unaerated for hours, so its oxygen is already low and its ammonia already rising — that is a few hours, not a day. A bucket of dechlorinated water with an air stone is days rather than hours, and an uncycled container of any kind means daily water changes from the first morning, because there is no filter doing the work. Do not feed for the first day; a fish that has been in a bag does not need food, and uneaten food in unfiltered water is the fastest way to make the situation worse.

Expect the first fortnight to be the dangerous one. Fish acquired this way have usually come from crowded holding systems, often already carrying something, and the early losses people blame on themselves were frequently set in motion before the bag was handed over. That is not a reason to be casual; it is a reason not to read an early death as proof you cannot keep fish.

Read the beginners guide and accept the underlying fact: a fish acquired for nothing needs exactly the same aquarium as one chosen deliberately. That is the awkward part, and it is the whole reason this site exists.

And the fork worth naming, because nobody else will. A goldfish is a fifteen-year animal that needs 75 litres and a weekly routine. If that is not something you can take on — not today, but at all — the answer is not a bowl and good intentions. It is to keep the fish alive and stable in the meantime and find it a keeper who already has the tank, which rehoming a goldfish sets out properly, including the routes that actually work and the one advert wording that attracts the wrong sort of reply. Deciding that in the first week is a better outcome for the animal than deciding it in the second year.

Buying online

Reputable specialist breeders pack properly, ship on oxygen, avoid shipping into weekends and guarantee live arrival, and the fish are frequently in better condition than general pet shop stock because they were bred rather than imported.

What to avoid is a listing with no named breeder, no description of packing, and no guarantee. And whatever the source, an online purchase goes into quarantine as a matter of course — you have had no opportunity to look at the fish, the shop system or the other stock, so the inspection this page is built around simply did not happen.