Appetite: the clearest good sign
An elderly goldfish who still enjoys life announces it at feeding time. He knows the routine, comes to his feeding place when he expects you, and takes food with real enthusiasm rather than mouthing it and drifting away. He may be slower to arrive and easier to outpace, but the intent is unmistakable — he wants the meal. As long as that eagerness holds, most other age changes are just age. The contrast worth acting on is not a smaller appetite but a lost one: food inspected and ignored by a fish who has always fed keenly is a change; a hearty old fish taking his time is simply an old fish at dinner.
Curiosity and recognition
Contented goldfish are nosy. An old fish enjoying his days still investigates — a stone moved during cleaning, a new plant, your hand in the water, your face at the glass he has known for years. Goldfish learn their people, and the old fish who still orients to you, follows movement outside the tank, and checks out anything novel is showing you his mind is engaged with his world. Curiosity fades before appetite in some fish, so weigh the two together: eating plus exploring is a good life; eating mechanically, with no interest in anything around him, is a dimmer picture.
Movement that's slower, not struggling
Age slows goldfish without making them miserable. Longer rests, a preference for the quiet end of the tank or pond, fewer circuits of his territory — all normal, provided the movement he does make looks easy: level swimming, unhurried turns, no effort needed to hold his place. What you are distinguishing is slowness from struggle. A fish labouring to stay upright, resting because swimming exhausts him within moments, or wedging himself into a corner to escape the current is not enjoying a quiet retirement — that is a fish coping, and the balance of his days has changed.
The social side and the comfort of routine
Goldfish are social, and an old fish still placed within his group — resting near companions, joining the general drift toward food, neither bullying nor bullied — is showing you he still belongs in his world. Watch that tankmates haven't started treating him as an obstacle: being shouldered aside at every feeding by younger fish is a quality-of-life problem you can fix with feeding arrangements, not a verdict on his life. Routine itself comforts him: the same feeding times, the same familiar layout, your same face. An old goldfish's good life is mostly the life he already knows, continuing to work for him.
Keeping a good-days record
Because decline is gradual, judge him against written notes, not against your memory of his prime. A short weekly line — ate keenly, cruised the tank twice, rested most of the afternoon — builds the honest picture: mostly good days with the odd quiet one is a life still being enjoyed, while a run of weeks in which appetite and curiosity both thin out is the pattern that should start a conversation with your vet. The record also protects you from the two worst judges in these decisions, panic and guilt, both of which read a single afternoon as a verdict.
Those weekly good-day notes deserve to live with the rest of his story, and KinsLe's Pet Passport keeps them alongside his health records and history, so the shape of his later life is documented when you need it.