Test first, then write down the 'before' picture
The most valuable line in the entry is the one written before your hands get wet. Take your readings first — temperature, and ammonia, nitrite, nitrate and pH on your testing days — because a nitrate result only means something compared with the change you are about to make. Note how the tank looked before you started: algae on the glass, mulm in the gravel corners, plants lifting or yellowing, the waterline dropped by evaporation. This 'before' snapshot is what later entries get judged against; without it, a keeper cannot tell whether the filter clean improved anything or whether the algae problem is quietly winning despite weekly effort.
What the job itself involved, in practical detail
Record how much water came out — a quarter, a third, half the tank — and how you know: a marked bucket or a line on the glass beats a guess. Note whether the replacement water was temperature-matched and treated with conditioner before it went in, and what you actually did with your hands: gravel siphoned in which areas, which sponges rinsed in removed tank water, which media replaced rather than rinsed, impeller checked, intake cleared, glass scraped, plants trimmed and replanted. Filter details matter most here, because media has a lifespan memory cannot track — the entry that says a sponge was replaced, or that a particular basket was rinsed this time and the other left alone, is the entry that prevents the classic mistake of washing everything at once and crashing the cycle.
The 'after' picture: fish behaviour and readings
Close the entry an hour or so later, once the tank has settled. Note how the fish responded — feeding eagerly at the next meal, exploring the trimmed plants, or sulking in a corner longer than usual after a big clean. Record the temperature once the heater has restabilised and, on days you test, the follow-up readings that show what the change achieved. If something looked wrong during the job — media crumbling, a cracked impeller shaft, a fish with a mark you had not seen before — write it here while it is in front of you, with what you decided to do about it or to watch.
Small notes that pay off months later
Leave one line for the miscellaneous: the bulb that flickered, the snail population creeping up, the plant species that melts every time you prune it, the reminder that the background filter sponge is due replacement soon. These asides look trivial in the week they are written and become the answer to a later question — when did the light start failing, when did the snails arrive, how old is this media exactly. Read back over a run of entries and your tank's rhythm appears: how fast nitrate climbs between changes, which jobs can stretch and which cannot. KinsLe's Pet Passport keeps the tank's records together in one portable packet, so the whole maintenance history — readings, jobs and asides — travels as one file whenever the tank is discussed, sold or handed to a new keeper.