Rescue the photographs first
A fish who has been in the family for years has been photographed far more than anyone realises — on old phones, in the background of birthday pictures, in children's snapshots of the tank, in the camera roll of a relative who visited once. Ask around before you assume the record is thin. Gather the best of them, put them into rough order from young fish to old, and pick clear shots that show how he changed: the deep orange youngster, the paler elder, the growth of him against the same ornament across the years. Photographs anchor everything else you record, because they prompt the memories that no one would have thought to write down unprompted.
The milestones only your family knows
No photograph explains itself, so write the captions while the people who know are there to be asked. How he arrived — won, rescued, chosen by a child who is now grown. The tanks he has lived in and the moves he survived. The companions he shared the water with and outlived. The periods he was unwell and came back from, and what the family did. His habits, most of all: where he waits at feeding time, the corner that is his, the person whose arrival at the glass he always notices. These are the facts that make him this fish rather than any goldfish, and they are exactly the facts that disappear when the generation that remembers them stops being asked.
Write his story in your family's voice
Put the milestones together as a short life story, written the way your family actually talks about him. It does not need to be polished or long; a page of honest sentences beats an elegant paragraph that sounds like nobody you know. Include the funny and the ordinary — the name and how it was chosen, the disaster with the filter that everyone survived, the summer he lived in a temporary tank in the kitchen. Children and grandchildren who never met the young version of him will meet him properly through this, and writing it now, while he is still the main character, keeps the tone celebratory rather than mournful.
Keep adding while he is still here
Treat the record as living rather than finished. Add a photograph each season, note the small changes of his old age, record who in the family has taken over his feeding and how he responded. A history kept this way grows naturally into the thing a family wants later: when his long life does end, the commemoration is already made — complete, accurate and full of him — instead of something assembled hastily from fragments in a sad week, when half the phones have been replaced and the stories are already going soft at the edges.
One profile the whole family can keep
Scattered across phones and albums, a record this rich slowly thins out as devices change and people move. His profile in KinsLe Memorial Accounts keeps the photographs, the milestones and his story together in one place attached to the people who loved him, where family members near and far can visit it, add their own pictures and keep his place in the family's history permanent.