Most of the mysterious problems that show up in a healthy aquarium three weeks after a shopping trip were not caused by the tank. They were bought and carried home in a bag. Ich, velvet, internal parasites, and the slow wasting that people describe as "it just never really settled in" almost always arrive as passengers on a fish that looked fine under shop lighting and was not fine at all.
The ninety seconds you spend looking at a fish before you agree to buy it is the cheapest disease control available to you, and it is more effective than any medication you will buy afterwards. This guide is about what to actually look at in that ninety seconds — including the things most people never think to check, like the tank the fish is sitting in and the plumbing behind it.
Look at the shop before you look at the fish
Start one step further back than the animal you came for. Walk the row and look at every tank, not just the one you are interested in.
Dead fish in any tank is information. One dead fish in a busy shop is not damning on its own; several across multiple tanks, or a fish that has clearly been dead long enough to be discoloured, tells you that nobody is looking. Uneaten food rotting on the substrate says the same thing in a slower way.
Then look at the plumbing. Many retail systems run a whole wall of tanks on one shared filter and sump, which means every tank on that wall shares its water. If any tank on the system is visibly sick, treat the entire wall as sick, including the healthy-looking tank you wanted to buy from. Individually filtered tanks are a meaningfully better sign. If you cannot tell, ask — it is a normal question and a good shop will answer it plainly.
A blue or green tint to the water usually means the system is being medicated. That is not automatically bad; responsible shops medicate prophylactically. But it does mean something is being treated, and you should ask what.
Ask when they arrived
This is the single most useful question, and almost nobody asks it. Fish that arrived yesterday have been bagged, shipped, temperature-shocked and moved into new water within the last day or two. Their immune systems are suppressed and any parasite they carry has not yet had time to become visible. They will often look perfectly good and then break out with ich four days later — in your tank.
Ask the staff how long the fish has been in the shop. If the answer is "they came in this morning," ask them to hold it for you for a few days. A shop that is confident in its stock will agree without hesitation, and a shop that refuses has told you something useful for free. Waiting three or four days costs you nothing and removes most of the risk.
Breathing rate: the signal most people never check
Watch the gills. Rapid, laboured gill movement in a fish that is otherwise sitting still is one of the earliest and most reliable signs of trouble — gill parasites, poor water quality in the shop system, or a fish that is simply in distress. Compare it against other fish of the same species in the same tank rather than against some absolute standard, since baseline rates vary a great deal between species.
Two specific patterns are worth walking away from. Gasping at the surface, when the tank is not obviously oxygen-starved, suggests the gills are not working properly. And one-sided gill movement — where a fish is visibly working one gill cover much harder than the other, or holding one flared — often means a fluke or another gill parasite on that side.
Body shape and the belly
Look at the fish from the side and note its outline. A healthy fish has a smooth, convex profile. Two deviations matter.
A sunken belly, or a pinched, hollow look just behind the head along the back, points to either internal parasites or long-term underfeeding. This is common in wild-caught imports that have been in transit and not eating. It is one of the hardest conditions to reverse, because by the time the outline visibly changes, a lot of muscle has already gone. It is the most under-appreciated red flag in the shop, precisely because the fish is still swimming normally.
The opposite — a swollen body with scales standing out from it, so the fish looks slightly like a pinecone when viewed from above — is dropsy, which is a symptom of organ failure rather than a disease in itself. Do not buy that fish, and be cautious about the whole tank.
Fins, skin and the things that look like spots
Fins should be held open and carried away from the body. Clamped fins — held tight against the flanks, as if the fish is trying to be smaller — is a general distress signal and is worth taking seriously even when everything else looks fine.
Damage to fin edges needs interpreting rather than just noting. A clean split or a triangular chunk missing is almost certainly physical damage from a tankmate or from netting, and it heals. Edges that look ragged, milky, or eroded inward from the tip, sometimes with a white or reddened margin, are more likely fin rot, and that comes with the bacterial background that caused it. Our guide to ich and fin rot covers the distinction in more detail.
On the body, white spots the size of a grain of salt, scattered across the flanks and fins, are ich. Do not confuse them with the small white bumps that appear on the gill covers and pectoral fins of some male fish in breeding condition, which are harmless and concentrated on the head. A fine gold or rust-coloured dusting, best seen with a torch at an angle, is velvet and is considerably more serious. Fuzzy white cottony patches are fungal, usually growing on an existing injury. Red streaks in the fins or a red ulcer on the flank both indicate bacterial infection.
Eyes
Eyes should be clear and sit flush. Cloudiness across the surface suggests poor water quality or a bacterial issue. An eye bulging outward is popeye, which can be an injury on one side or a systemic infection when it is both. A sunken eye is a bad sign in the same category as a sunken belly — it usually means the fish has been going downhill for a while.
Behaviour worth watching for
Flashing — a fish darting sideways to rub its flank against rock, substrate or equipment — means something is irritating the skin or gills. One flash is nothing; repeated flashing from several fish in the tank is a parasite load.
Distinguish hiding from lethargy. Plenty of species are naturally shy, and a new arrival that retreats when you approach is behaving normally. A fish resting on the bottom propped on its pectoral fins, breathing hard, not reacting when the tank is disturbed, is a different thing entirely. Livebearers doing a stationary shimmy — rocking in place without going anywhere — are usually telling you about water quality or temperature in that system.
Buoyancy problems, where a fish struggles to stay level or floats up at the tail, are worth avoiding in a shop even though they are sometimes temporary and diet-related. Let someone else find out.
One more thing that is visible if you look: long, white, stringy faeces trailing from a fish, or lying on the substrate. That is a genuine indicator of internal parasites or an intestinal infection, and it applies to the whole tank rather than just the individual producing it.
Ask to see it eat
This is the strongest single test available to you and it takes thirty seconds. Ask the staff to drop in a pinch of food and watch. A healthy fish notices immediately and goes for it. A fish that ignores food, or approaches and then turns away, or takes food and spits it back out, is not well — and the reason may not be visible yet.
This test is particularly valuable for wild-caught species and for anything that arrived recently, because refusing food is often the first symptom in the chain. Most shops will happily do this. A shop that refuses outright has answered a different question for you. It is also worth checking what they are feeding, which connects to the practicalities in our article on how often to feed fish — a fish accustomed to live food alone can be difficult to transition.
Species-specific traps
Some categories deserve their own warnings.
Bettas are usually kept in individual cups or small dividers, so the visual cues change. Look for clamped, curled or heavily damaged fins, and for a fish sitting listlessly at the bottom of the cup. Our betta care guide covers what a healthy specimen should look like. Fancy goldfish are frequently sold from crowded tanks and are prone to buoyancy issues; the background in our guide to keeping goldfish is worth reading before you go.
Tanks labelled "assorted" that hold several similar-looking species mixed together are a warning sign in themselves — it usually means stock has been consolidated, and consolidated stock means mixed sources sharing water. The same applies when a shop cannot tell you what a fish actually is. If you cannot identify it, you cannot check whether it fits the rules in our fish compatibility guide or the stocking maths in how many fish can I put in my tank.
Then there are dyed and injected fish — painted glassfish, artificially coloured parrot cichlids, and similar. The colour is applied by injection or dipping, the process kills a large proportion of the fish subjected to it, and the survivors have compromised immune systems. There is no good version of this. Do not buy them, and the demand goes away.
For high-value fish the calculation changes again, because the stakes are higher and the paperwork can matter. Our guides to arowana care and to buying an arowana in the UAE and CITES documentation cover what to verify before money changes hands. Similar care is worth taking with African cichlids, where mislabelled species are extremely common and an incorrect identification wrecks a stocking plan.
Invertebrates need their own checklist. Shrimp should be actively grazing rather than sitting motionless, and a shrimp with a visible white ring around the middle of its body is in trouble. Our notes on cherry shrimp and on algae eaters that actually work cover what healthy specimens look like. For bottom dwellers, check barbels specifically — eroded or missing barbels on a corydoras, as discussed in our corydoras guide, indicate sharp substrate or poor water in the system it came from.
Bagging, the drive home, and the summer problem
How the fish is bagged matters. There should be roughly one part water to two parts air, the bag should be doubled for anything with spines, and it should go into an opaque bag or box — darkness calms fish considerably during transport.
Then there is the specifically local problem: a car in a Dubai summer. A bag left in a boot or on a back seat while you finish your errands can climb well past any survivable temperature within twenty minutes, and no amount of careful selection at the shop survives that. Make the fish shop your last stop, put the bag in the air-conditioned cabin rather than the boot, and drive home. If the drive is long, an insulated cooler bag is genuinely worth carrying.
Once home, acclimate properly rather than pouring the bag in — the method is in our guide to acclimating fish after delivery. This matters at least as much for fish bought in person as for fish delivered.
And then quarantine anyway
Everything above reduces risk. None of it eliminates risk, because a fish incubating an infection looks exactly like a fish that is not. A separate quarantine tank for two to four weeks is the only thing that actually protects an established display tank, and it is the step almost everyone skips and later regrets. Our guide to quarantining new fish covers how to run one cheaply.
This applies with even more force to marine fish, which are mostly imported and have usually had a much longer journey, as covered in our companion piece on what freshwater keepers should know before starting a saltwater tank.
Quick answers
What is the fastest single check? Watch it eat, and count its breathing. Those two together catch the majority of problems that are visible at all.
Is one dead fish in the tank a dealbreaker? Not by itself. Dead fish across multiple tanks, or a shared filtration system with sick fish anywhere on it, is.
Should I buy fish that arrived today? Preferably not. Ask the shop to hold them three or four days. Most will.
The fish looks fine but the tank is crowded — is that a problem? Crowding raises stress and pathogen load even when nothing is visibly wrong. It is a reason to ask more questions rather than an automatic no.
Can I skip quarantine if the fish looked perfect? This is exactly the situation quarantine exists for. A fish incubating ich looks perfect right up until it does not.
What if I already added a fish and now the tank is sick? Work through the diagnostic order in why are my fish dying and confirm your water parameters first using our guide to reading test kit numbers — treating for a disease when the real problem is ammonia makes things worse.
The bottom line
Picking fish well is not about expertise, it is about slowing down. Look at the whole shop, not just the fish. Ask when it arrived and ask to watch it eat. Check breathing, belly outline, fins and eyes, in that order. Walk away from anything with a sunken profile, laboured breathing, or white stringy waste, no matter how good the price is — and be equally willing to walk away from a fish you came specifically to buy.
Then quarantine it anyway. A tank that has been running well for a year has no natural defence against something you introduce to it, and the cost of one careless purchase is measured in the fish you already own. If you are still building your stocking list, our guide to the best beginner fish for UAE tanks and the cycling primer in how to cycle a new aquarium are the right places to start, well before the first shopping trip.