Names first, and names that never change
Mix-ups start with identity, so name every tank in a way that cannot drift: by room and size, or by its permanent character, and use that exact name on the record, on the equipment labels and in conversation. The big tank and the other tank are names that swap meaning depending on who is speaking; the lounge community and the spare-room quarantine never do. Put a small label carrying the name on the tank itself, because the record you need at midnight is the one you can match to glass in front of you. If a tank is rescaped, the name stays, since identity follows the aquarium and its history, not its current decoration.
One record per tank, identical in shape
Keep a separate record for each tank and build every record in the same order: stock, water targets and results, feeding, equipment, maintenance log. Identical structure is a safety device, because your eye learns where a fact lives and notices when an entry sits in the wrong file by its shape alone. Resist the convenience of one combined fish-room log with entries for whichever tank was serviced; combined logs read smoothly and corrupt quietly, since a single mistyped tank name poisons two histories at once. Cross-references are fine, such as noting fry moved from the breeding tank to the grow-out, written into both records at the moment it happens.
Log at the tank, before you move on
The golden rule of multi-tank records is that the entry is written standing at the tank, straight after the test or water change, before hands touch the next aquarium. Most errors are not carelessness but sequence: three tanks serviced in a row, three sets of results held in the head, and one set written into the wrong page an hour later. A phone or notebook that travels with the bucket solves it, as does a simple household rule that a tank is not finished until its record is. Back-to-back entries in the same hand and the same numbers are a warning sign worth double-checking on the spot.
Make the tanks themselves carry the system
Support the paperwork physically: colour-coded labels, nets and siphons per tank where disease control matters, with the same colour noted on that tank's record. Keep test reagents and food in one station but write results directly into the right record rather than on a shared pad that gets transcribed later, since transcription is where figures quietly transpose. Photographs help more than descriptions; a current photo at the head of each record settles at a glance whether you have opened the right file, and updating it after a rescape keeps that safeguard honest.
Share the records with the whole household
Multi-tank homes rarely have one pair of hands, and records fail when a second person services a tank and logs nowhere, or logs into whichever file they find first. Give everyone who touches the tanks access to the same set of records, with the same names, so a partner's Saturday water change lands in the same history as your Wednesday one. The relatives you choose can open the profile of the tank they are servicing in KinsLe Family Sharing, so a second pair of hands meets the right aquarium on screen before a net ever touches water.