What a useful treatment entry records
A log earns its keep when it captures the story around the treatment, not just the treatment itself. Open each entry with the date and a plain description of what you can see: which fish, how many, and the exact signs, whether that is white grains on fins, a patch of raised scales or a fish sitting on the gravel breathing hard. Follow it immediately with that day's water readings, because a treatment started during an unnoticed ammonia rise will fail, and the log should be able to tell you later whether it did. Then record what you used, when each application went in, and practical details future-you will need: whether filter carbon came out, whether lights were dimmed, how much water was changed along the way.
Set the log up before you need it
The worst moment to design a record system is while a fish is gasping, so keep the structure ready in advance. A notebook that lives beside the tank, a note on your phone, a page per tank: the medium matters far less than having ruled space for the same fields every time, so entries can be compared months apart. Keep one entry per episode rather than a continuous diary, with a clear start date and a blank outcome line waiting at the bottom. Note also which tank and which filter the entry belongs to if you run more than one setup, because treatments and equipment migrate between tanks in memory long before they do in fact.
Write one honest line a day while treatment runs
Daily notes are where the log becomes genuinely diagnostic. A single line is enough: ate eagerly, still hiding, spots unchanged, two more fish showing specks, breathing easier. Those lines capture the trajectory, and trajectory is what separates a treatment that is working slowly from one that is failing while you hope. Record water readings again mid-course, since many treatments stress the filter bacteria and the water can slide while your attention is on the fish. If a fish dies, write that down too, without flinching; an honest mortality line is often the detail that changes what you do next time.
Read the log back like a detective
Once an episode closes with its end date and outcome, the log starts paying rent. Flick back through old entries and patterns surface that memory alone would never hold: outbreaks that always follow new arrivals point to a quarantine gap, problems that cluster after filter cleans point to technique, a species that falls ill every time points to a mismatch with your water. When you do consult an aquatic vet or an experienced fish-health adviser, this record is the consultation: dates, readings, what was tried and what happened, instead of a vague account of a bottle used sometime last spring.
Keep the log where the tank's other records live
A treatment log separated from the tank's test history, stock list and water recipes forces you to reconstruct the picture from fragments exactly when time is short. Storing the log in the tank's KinsLe Pet Passport keeps every treatment alongside the water tests that explain it and the quarantine notes that may have prevented it, so the whole health story of the tank reads in one place and travels intact if the tank and its fish ever move to a new keeper.