The confusion starts with filters having no names
Ask a keeper with three tanks and five filters when the carbon in "the big filter" was changed and you will usually get a pause, a guess, and a shrug. The problem is not carelessness — it is that the filters were never given identities, so their histories blur into one. Start the log by naming every filter distinctly: the tank name plus a position or number, written the same way every time, and ideally written on a label or a piece of tape on the filter itself. From then on, every media entry belongs to a named filter, and "when did I change that?" becomes a lookup instead of an archaeological dig.
One media line per type, per filter
Inside each filter's entry, keep a separate line for each kind of media it holds, because they live on different clocks. Chemical media — activated carbon and the resin products used to polish water — exhaust as they adsorb, and their working life runs out whether or not the filter has been opened since. Mechanical media like sponges and floss are rinsed or replaced on condition. Biological media should barely ever be replaced at all, and then only in part. A single line per media type, with the date it went in and a running list of dates it was serviced, keeps those clocks separate: the log shows the carbon due for renewal even though the sponge was only rinsed last week and the ceramic media has not been touched in a year, quite rightly.
Write the install date on the media's home, too
Belt and braces: when fresh chemical media goes into a filter, write the date on a small piece of tape stuck to the filter body or the media basket itself. Then the log and the hardware agree, and a discrepancy between them is immediately visible — if the tape says one date and the log another, one of them missed an entry, and you find that out at the filter rather than three months later. This is especially valuable with chemical media, whose exhaustion is invisible: carbon does not change colour or clog when it is spent, so the date is the only evidence you will ever have.
Stagger swaps so the tank never loses ground
The log's second job is scheduling. Changing every chemical media in every filter on the same weekend, or ripping through all filters at once on a single tank, strips out adsorptive capacity and disturbs bacterial colonies in one blow. Use the log to spread the work: filters take their turns a week or two apart, and on a tank running two filters, never service both in the same session unless something has gone wrong. Mark planned swap dates in the log when you complete a change — the next due date written at the moment the fresh media goes in — so the schedule is always looking forward rather than relying on you to remember to check.
A quick monthly scan keeps the system honest
Once a month, read down the log and mark anything due or overdue; chemical media running two months past its change date is not a catastrophe, but it is no longer doing the job you put it in for, and water polish and odour control fade so gradually that only the dates will tell you. Kept in a KinsLe Pet Passport alongside the equipment register, the media log also survives a house move or a change of keeper: the next person inherits not just filters, but the full service history of what is inside them.