Plecos are the hardest fish in the shop to judge, because a healthy one spends the day clamped to wood or glass in a shadowed corner, looking exactly like a sick one. The trick is to get the fish where you can see it: ask staff to catch it into a clear viewing container or let you watch it on the glass, and take your time. Three checks — belly, fins and breathing — plus how the fish responds, will tell you most of what a seller's reassurance cannot.
The belly: rounded, not hollow, not swollen
Turn your attention to the underside while the pleco is on glass or in the container. A well-fed pleco has a belly that is flat to gently rounded, with no sharp outline of bone at the edges. A sunken, hollow belly — the underside caving inward — is the commonest quiet problem in shop plecos: it usually means the fish has not been competing successfully for food, has been in transit and unfed, or has an internal problem, and starved plecos are hard to bring back. The opposite extreme is not reassuring either: a distinctly swollen, rounded belly on a fish that is not a well-fed female can signal internal trouble. Look along the body too — a healthy fish is filled out behind the head, not pinched or triangular.
The fins: whole, spread and clean
Fins show stress early. You want a dorsal fin that rises and spreads when the fish moves, a tail fin with a clean, even edge, and pectorals that are complete and symmetrical. Walk away from frayed or eroded edges, fins held clamped tight to the body, white spots scattered over fins and skin, fuzzy or cottony patches, redness at the fin bases, or fins that look as if something has been nibbling them down. Also check the skin between: smooth, with its normal pattern clear — no pale grey film, raised bumps, or sores, and eyes that are clear and bright rather than cloudy or sunken.
Breathing and behaviour: slow, even, strong
Watch the gill covers for a full minute. A settled pleco breathes slowly and evenly; rapid, laboured pumping, one gill working harder than the other, or a fish hanging at the surface in the shop tank all argue for choosing another individual. Then test the response: a healthy pleco grips. It should hold the glass or wood firmly with its sucker mouth, sit solidly rather than drifting, and when a shadow passes or the container is tapped gently, it should dart or clamp down decisively — a listless fish that allows itself to be moved without protest is telling you something is wrong.
Shop questions, and starting the record
Ask how long the fish has been in stock — very recent arrivals are still stressed and unfed — and whether it has been seen eating; staff who feed plecos after lights dim can often show you a feeding response at closing time. If everything checks out, note what you saw on the day you buy: belly shape, fin condition, breathing rate and what the fish was eating in the shop. Those notes become the baseline you compare against during the vulnerable first fortnight at home, and KinsLe's Pet Passport is the place to keep them — belly, fin and breathing notes as the opening entry in your pleco's portable health packet, ready for a vet or a future keeper to read at a glance.