Sort by tank first, then by kind
Receipts become useless the moment they merge into one household paper pile, so give each tank its own file and keep two sections inside it: livestock and equipment. A filter receipt for the quarantine tank and one for the display tank may look identical in a drawer, yet they belong to different machines with different ages and guarantees. When a shop asks which tank a fish came from, or a manufacturer asks when a heater was bought, the answer should be a thirty-second search, not an excavation. Keep the file where the tanks are, not in general household paperwork, so it is at hand during a fault rather than across the house.
Capture the facts before the paper fades
Thermal till receipts fade to blank within a surprisingly short time, and a faded slip proves nothing. Photograph or scan every receipt on the day of purchase and keep the image beside the original, and transcribe the essentials onto the record: what was bought, where, the amount paid, and the length of any guarantee. For livestock, add the species, number and the tank they entered, since a shop's own return or health window usually runs from the purchase date and applies to specific fish. Email confirmations from online orders deserve the same treatment, saved as files rather than left to sink in an inbox.
Match guarantees to the equipment list
Your tank profile already lists the equipment fitted; the guarantee file should mirror that list so each machine's paperwork can be found from its entry. Note beside every item when its guarantee runs out, judged by the purchase date on its receipt, and keep manuals with the matching guarantee rather than in a separate drawer. When a part is replaced under a claim, file the replacement's paperwork with the original and mark the swap, so a second claim is not argued against the wrong purchase. Spares bought but not yet fitted belong in the same file, labelled as unused, so their guarantees are not wasted through forgetfulness.
Livestock paperwork has its own uses
Receipts for fish, shrimp and plants settle more than returns. They build the stocking history of the tank: what arrived when, from which shop, which matters when you are tracing the source of a problem that followed a new arrival. Keep quarantine notes with the matching purchase record, and the story of each batch stays complete. For expensive or rare livestock, the receipts also document what you actually hold, which matters for your own records if the collection is ever sold, split or handed on with the tank.
Review the file once in a while
Once or twice a year, thin the file out: discard guarantees for equipment long gone, and check which current items are approaching the end of their cover, because a heater near the end of its guarantee deserves watching and perhaps a planned replacement. File everything against the tank in its KinsLe Pet Passport and the receipts, guarantee dates and stocking purchases sit permanently beside the equipment and livestock lists they prove, ready the day a claim or a question arrives.