What changes along the substrate
An aging corydoras catfish works the bottom at a gentler pace than the fish it used to be. A young cory shuttles nose-down across the sand in near-constant motion, barbels sweeping, pausing only to gulp what it finds, while an old one settles into a resting cluster for long spells and makes shorter foraging trips between rests. At feeding time the difference costs it meals: sinking wafers land, the keenest shoal members pile in, and the senior arrives as the wafer softens into fragments. It still eats, but it eats the tail end of every feeding, and across weeks that shortfall reshapes its body.
Why an old cory's belly slims or swells
Most aging corydoras catfish lose a little condition because effort goes up as reward goes down — longer rests, shorter searches, and leftovers instead of first pickings. The belly between the bony scutes flattens, the plump triangle a healthy cory shows from above narrows, and worn barbels on very old fish make finding buried food harder still, compounding the problem. The opposite case also exists: a senior that has learned to rest exactly where food lands can grow noticeably round while barely swimming, and that plumpness is sediment and wafers, not fitness. Distinguish the two by behaviour as much as shape — a thin forager needs feeding help, while a round sitter needs its windfall shared out.
Feeding the slowest member of the shoal
Feed an aging corydoras catfish at its resting place, after the tank lights dim. Break a wafer into pieces and place one piece beside the cluster where the seniors rest, scattering the rest widely so the young shoal cannot guard it all. Small sinking foods that spread — micro-pellets rained across a broad area of sand — give a slow cory many chances instead of one contested pile. Keep the substrate soft and clean under the resting spots, because an old cory sitting still for hours on gritty or dirty gravel risks worn barbels and sore skin that further dull its appetite.
Reading plates, barbels and belly
An aging corydoras catfish in fair condition keeps a belly that rounds out between the plates after feeding and barbels of even length that sweep actively when it does forage. Shortened, uneven barbels, a belly that stays flat after meals, and longer motionless spells week on week form the pattern of a fish eating too little. Watch the resting cluster as a group: corydoras are social, and one old fish consistently choosing to rest apart from the shoal, away from both company and food, is telling you something the scales cannot.
When bottom feeding fades too far
An aging corydoras catfish that stops coming out to the evening wafer, breathes rapidly at rest, or shows redness around worn barbels needs an aquatic vet or experienced fish-health advisor, and sooner if several shoal members change together. Test the water near the bottom and record the results, because waste collects in the substrate layer where corys live and a whole group slimming at once usually points there rather than to age. Note which foods the senior still rouses for, since an old cory that perks up for frozen bloodworm but ignores wafers is making a preference clear, not refusing food entirely. Logging those water readings, barbel checks and wafer responses in KinsLe's Pet Passport gives whoever advises you the full bottom-dweller picture.