Why a treatment deserves its own log page
Treating a tank usually happens under stress, and stressed memory is unreliable: three days into a course, few keepers can say with confidence which days they treated, when the big water change happened, or whether the fish stopped eating before the treatment or after it started. A medication log answers those questions while they still matter. It also protects the fish. If the first treatment does not work and you need advice from a vet or a good aquatic shop, the first thing they will ask is what has already been used, for how long, and what the water was doing at the time — because some treatments cannot follow others safely, and a repeated course of the same product is a very different decision from a first one.
Set the page up before the first treatment goes in
Start the log on the day you decide to treat, and write the header before anything touches the water. Record the tank, the date, and the symptoms in plain observable terms — white spots on the fins, a fish flicking against rockwork, clamped fins and no interest in food — rather than the name of a disease you are guessing at. Copy the product name exactly as printed on its bottle, note the stated course length from its label, and run a full water test the same day: temperature, pH, ammonia, nitrite and nitrate all go in the header. That test matters because poor water causes symptoms that look like disease and defeats treatments aimed elsewhere; if your ammonia was raised when you started, the log should say so.
One honest line for every day of the course
Daily entries can be short, but they must be daily. Note that the day's treatment was added, any water you changed and roughly how much, whether carbon or other chemical media is out of the filter, and two observations: are the fish eating, and are the visible symptoms spreading, holding or fading? Note losses the day they happen. This running line is what turns a vague worry into a pattern you or an adviser can read — a fish that improves on day two and relapses on day four is telling you something different from one that never improves at all, and you will only see it if the entries exist.
Water changes and filter media belong in the same log
The two details keepers most often forget are the ones that change how a treatment behaves. Many courses are interrupted or diluted by water changes, so every change during treatment gets logged with its size and whether treatment was re-added to the fresh water according to the label. Chemical filtration matters just as much: carbon and similar media pull treatments back out of the water, so the log should show the date the media came out and the date it went back in at the end. If a treatment appears to fail, these two columns of the log are usually where the explanation is hiding.
Close the log with an outcome, not just an end date
When the course finishes, write the ending properly: symptoms cleared or not, losses, how the fish are feeding a few days later, and anything you would do differently. That closing note is the part your future self will actually use, because tanks tend to meet the same problems twice. Keeping the finished log in a KinsLe Pet Passport, alongside the tank's water-test history and stocking record, means the next problem starts from evidence instead of recollection — and anyone advising you can see the whole history in one place.