Buying a fish: judging the shop tank, the health check, and adult size

Quick answer

Fins held open and fully spread, clear eyes, even breathing. This is the baseline to compare a shop fish against.

Most fish that die in home aquariums were already in trouble when they were bought, or were the wrong fish for the tank before anyone put water in it.

Two things decide the outcome, and neither is the individual fish's colour. The first is whether the shop's tanks share one filtration system, because on a shared system a disease in any tank is a disease in every tank. The second is the fish's adult size and social needs, which are rarely printed on the label and almost never match the size of the animal in the bag.

• Look at every tank on the rack, not the one you want. Shared plumbing means shared pathogens.
• Ask to watch the fish eat. A fish that will not feed in the shop very rarely starts at home.
• Ask for adult size and minimum footprint before you ask the price.
• Clamped fins are the earliest general sign that something is wrong, in almost every species.
• Buy the quarantine tank before you buy the fish, not after the first outbreak.

:::danger
Never release an unwanted aquarium fish or plant into a pond, river, canal, drain or the sea.

Released aquarium species have established damaging populations on every inhabited continent, and a single fish can carry parasites that native populations have no resistance to. In many countries it is also a criminal offence. The route out of a fish you can no longer keep is a rehoming group, a specialist shop, or humane euthanasia by a method your vet advises. Never the outside world.
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Decide in 60 seconds

Why the shop's plumbing matters more than the fish

Walk into any aquatic shop and look behind the display tanks rather than into them. If you can see a standpipe or an overflow slot at the back of each tank, and a single large filter or sump under the rack, that rack is one body of water divided by glass.

This is efficient and it is not in itself bad practice. It is, however, the single most important fact about your purchase. A parasite, a bacterial infection or a fungus in the bottom left tank has been circulating through the tank you are buying from since it arrived. The fish you take home can be incubating something that no fish on the rack is yet showing.

So the assessment is of the system, not the animal. Scan every tank. You are looking for dead fish, fish hanging at the surface, fish sitting on the bottom, white spots on anything, and cotton-like growths. One sick tank on a shared system disqualifies the system for that day.

Shops that run individually filtered tanks, or that keep quarantine stock separate from sale stock, are worth paying more at. Ask. The answer, and the willingness to answer, tells you a great deal.

Dead fish left in a display tank tell you something separate and equally useful: how often anyone looks. A tank nobody checks is a tank where an outbreak runs for a week before it is noticed.

The head-to-tail check, through the glass

You cannot handle the fish, and you should not want to. Everything below is visible from outside, and it takes about two minutes per fish.

Fins.

Held open and spread while swimming. Clamped fins, pressed flat against the body, are the earliest general distress sign in most species and mean water quality, parasites or infection until proven otherwise. Look at the fin edges: they should be clean and translucent. A ragged, white-edged or red-edged margin is fin rot or damage.

Gill covers.

Even, unhurried movement, the same on both sides. Rapid gilling in a shop tank usually means oxygen or ammonia rather than disease, but it means the system is under strain either way. One gill cover flared open, or moving alone, is a gill parasite or a lodged object.

Eyes.

Clear and gently convex. Cloudy eyes suggest water quality or infection. Bulging suggests popeye or internal disease. Sunken eyes are a wasting sign and, in an otherwise well-fed-looking fish, one of the more sinister things you can see.

Body outline, from the side and from above.

Symmetrical, with no swelling and no dents. Look particularly at the profile just behind the head. A knife-edge or pinched-in look there means long-term wasting, not a slim build, and it is the classic look of chronic internal disease. Scales standing out from the body, so the fish looks like a pine cone from above, is dropsy and the fish is very unwell.

Spine.

Straight from above and from the side. A kink or an S-curve may be developmental, or it may be mycobacterial infection, which is untreatable and can also affect people through cuts on the hands.

Skin and scales.

No white pinhead spots, no fine gold or rust-coloured dust in angled light, no cotton-wool tufts, no red streaks, no ulcers, no missing scales, no patches of thickened milky slime.

Faeces.

If you can see a fish trailing white, stringy faeces, it is either not eating or carrying internal parasites. Both matter.

Behaviour.

Rubbing or dashing against decor, shimmying on the spot without going anywhere, spiralling, hanging in a corner, or sitting on the substrate when the species is a mid-water swimmer. A shoaling fish that will not join its group is telling you something.

The feeding test.

Ask a member of staff to feed the tank while you watch. Healthy fish of almost every commonly sold species come up hard for food. A fish that ignores it, mouths it and spits it out, or approaches and turns away, is not a fish to take home. This test is worth all the others combined, and it costs the shop thirty seconds.

The adult-size problem

A young goldfish in a small tank. It suits the tank today. It will not suit it in a year, and goldfish routinely live for over a decade.

The most common way a fish's life goes wrong is not disease. It is that the animal was sold at a size that fitted, and grew.

The myth that keeps this going is that fish grow to the size of their tank. What actually happens is worse and less visible. In a confined space, skeletal and muscular growth slows while the internal organs continue to develop on their own schedule. The result is a stunted fish with compressed organs, a deformed spine, chronic stress and a fraction of its normal lifespan. It is not a smaller healthy fish. It is a sick one that looks tidy.

In poorly maintained tanks there is a second mechanism: growth-inhibiting compounds accumulate in water that is not changed enough, suppressing growth further. That is a symptom of a failing tank, not a feature of it.

Species that are routinely sold small and are almost never rehomed successfully include the common plecostomus, which is commonly cited as reaching forty to fifty centimetres; the iridescent or pangasius shark; the red-tailed catfish; bala sharks; tinfoil barbs; silver dollars; Chinese algae eaters; oscars; and common goldfish and koi, which are pond fish sold as tank fish almost everywhere in the world.

Ask three questions before you buy anything:

What is its adult length? What is the minimum tank footprint, meaning length and width rather than volume, because a fish that swims horizontally needs turning room more than it needs depth? And what does it need to live with?

If the answer to any of them is a shrug, take the Latin name, leave, and look it up. Any shop worth returning to will respect that.

Groups, not individuals

A large share of commonly sold aquarium fish are obligate shoalers: most tetras, most rasboras, danios, corydoras catfish and many loaches. They are sold singly because they are sold by the fish.

Kept alone or in pairs, a shoaling fish does not adapt. It becomes pale, spends its time hidden, feeds poorly, and is far more likely to nip the fins of anything else in the tank, because the behaviours that would be spent on the group have nowhere to go. It also dies younger.

The planning question is therefore never "can I fit one of these". It is "can I fit the group this species needs, for its adult size, alongside everything else I intend to keep".

Conversely, some species must be kept alone or in carefully chosen combinations, and a shop will happily sell you two males of a territorial species that will fight to the death. If a species is territorial, ask specifically how many you can keep and in what sex ratio.

Before you buy: is your tank actually ready

Net, conditioner, measuring jug, gravel vacuum. All of this belongs in the house before the first fish does.

The journey home

Know your tank's real temperature before the bag comes home, and match it against the temperature the fish has been kept at.

The bag is a closed system that gets worse by the minute, and there is one piece of chemistry that changes what you should do with it.

As the fish respires, carbon dioxide builds in the bag water and pushes the pH down. Ammonia is also building, but at a low pH almost all of it stays in the ionised form, which is comparatively harmless. The fish is therefore reasonably safe in a sealed, dark, still bag for a few hours.

Open that bag, and the carbon dioxide escapes within minutes. The pH climbs back up, and the ammonia that has been accumulating converts to its un-ionised, toxic form. This is why fish sometimes die during a long, careful acclimation after surviving a long journey perfectly well.

The practical consequences:

Drive straight home. Keep the bag upright, dark and insulated. Do not open it in the car to check.

For a short journey, floating the sealed bag to equalise temperature and then adding small amounts of tank water over twenty to thirty minutes is reasonable.

For a long journey, or any bag that has been sealed for hours, temperature-match the sealed bag, then net the fish out into the tank promptly rather than dripping tank water into bag water for an hour. Never tip shop water into your aquarium either way.

In a heatwave or a freeze, ask for a polystyrene box or bring a cool box. A shop that declines to sell in extreme weather is showing you good practice, not poor service.

What never to do

Do not buy a fish to cycle a tank. Fish-in cycling is still recommended in shops and it means deliberately exposing an animal to ammonia and nitrite burns for weeks.

Do not use the one-inch-of-fish-per-gallon rule. It ignores adult size, body mass, waste output, territory and swimming style, and it has probably killed more aquarium fish than any pathogen.

Do not buy a fish because it is the last one left and looks lonely. On a shared system, the last one left is frequently the survivor of something.

Do not buy on the same trip as your tank, on the same day you set it up, or on the same day you did a large water change.

Do not accept "they will be fine together" without the adult sizes. Compatibility at two centimetres tells you nothing about compatibility at twenty.

Do not skip quarantine because the fish looks well. Most of what you are quarantining against has an incubation period, which is the entire point.

Do not top the bag up with your own tank water at home and then put the whole lot in. Shop water goes down the sink.

The questions that get honest answers

Shops answer better questions better. These five are worth memorising.

"Is this rack on one filtration system?"

Tells you whether you are assessing one tank or twenty.

"How long has this fish been in the shop?"

A fish that arrived this morning has not been observed. Two weeks in stock is a far better sign than a bright colour.

"What is your water's hardness and pH?"

If the shop keeps soft-water fish in hard water, or the reverse, you now know what the fish is adapted to and how big a change your tank will be.

"What does it eat, and can I watch it feed?"

The behaviour test, plus a practical answer about whether you can supply the diet.

"What size does it reach and what does it need to live with?"

The question that prevents the rehoming problem five years from now.

When to get help

Contact an aquatic or exotics veterinarian, or an experienced local aquarist society, if a newly bought fish develops any of the following in its first month: white spots or a dusty gold sheen, cotton-like growths, open sores or red streaks, white stringy faeces, rapid or laboured breathing, a swollen body with raised scales, or a spinal curve that was not there when you bought it.

Go quickly if more than one fish is affected at once. Simultaneous illness in several fish points at the water or at something recently added, and both are fixable.

Bring these to the consultation: your tank dimensions and volume, how long it has been running, the filter type and when the media was last changed or cleaned, ammonia, nitrite, nitrate, pH, hardness and temperature readings taken today, a full stocking list with the dates each fish arrived, the shop or source, what you feed, and a short video of the affected fish swimming.

Expect to be asked about the water before you are asked about the fish. In aquarium medicine the water is the patient's environment, its lungs' interface and its toilet all at once, and the test results usually explain more than an examination of the animal does.

If a fish turns out to be a species you cannot house at adult size, act early rather than at crisis point. Specialist shops, aquarium societies and public aquaria sometimes take large fish, and a healthy juvenile is far easier to rehome than a stunted adult in a tank it can no longer turn around in.