Start with the story, in order
Fish-health staff think like detectives: what happened, in what order, over how long. Write the notes the day the sign appears, not the day of the visit, while the sequence is still exact in your mind. A clear account — three tetras stopped schooling, then one died, now a fourth hovers alone — gives staff something to work with; 'my fish are sick' gives them nothing. Name the species and numbers involved, and say whether one fish is affected or the whole tank.
Bring the water's numbers
Expect the first question to be about water, because most aquarium illness starts there. Recent readings for ammonia, nitrite, nitrate, pH, and temperature — ideally from the day of the visit and a few days before — let staff rule the commonest causes in or out immediately. If you do not have readings, bring a sample of tank water in a clean, rinsed container and say clearly that it is untested; a sample measured at the counter beats a guess. Note when you last changed water and how much.
Tank facts staff will ask about
Size, filtration, how long the setup has run, the full stock list, and feeding routine all shape what advice makes sense — a treatment safe in one setup can be wrong in another. Note the tank's age honestly, including whether it was fully cycled before fish went in. Also note how the tank is lit and for how long each day; light drives algae, and a bloom or a sudden die-off can be part of the story. If you have made changes, bring them: staff cannot factor in the new arrivals or the filter you replaced if they never hear about them.
Recent changes: the usual suspects
List anything new in the last month: fish, plants, decor, food, a different water conditioner, a deep-cleaned filter, a missed water change, a heatwave that pushed the temperature up. Also anything from outside the tank — aerosols or cleaning sprays used nearby, building work, a child's helpful extra feeding. Photographs and a short phone video of the behaviour you are describing are worth paragraphs: a shimmy, a scratch against decor, a way of holding a fin often identifies the problem family at a glance. If another household member feeds the fish, say so — double feeding is one of the commonest hidden causes staff uncover.
What good staff do with good notes
With a clear story and real numbers, staff can separate a water problem needing changes at home from a condition needing treatment, suggest the right next step in the right order, and tell you what to watch over the following days. They will also say when a case belongs with an aquatic vet instead — and your notes transfer with the fish, so nothing is lost in the referral.
The notes take five minutes to write at the tank, and keeping them with your test results in your tank's Pet Passport on KinsLe means the next visit — to the shop or a vet — starts from the whole timeline, not from memory under pressure.