Write a timeline, not a diary
The most useful note you can hand an aquatic vet or a good fish shop is a plain timeline of the problem. Start with when you first noticed something wrong and what exactly you saw: not eating, a white spot behind the fin, swimming in spirals, hiding in a plant it never used. Then follow the changes in order — what appeared next, what got better, what got worse — with the sequence kept honest even when it embarrasses you.
Vague worries are hard to diagnose from; specific observations are not. "Three tetras stopped schooling and hung under the filter outflow, then one developed a bent spine over the next two days" gives an expert a pattern to work with. Note which fish are affected and which are not, because a problem in one new fish, in every fish of one species, or in the whole tank at once points toward very different causes.
Record the water as numbers, not impressions
Fish live in their water, so most diagnostic conversations start there — and "the water seemed fine" closes that door. Write down the actual readings from your test kit each time you test during a problem: the waste compounds, acidity, and temperature as numbers, alongside when the last water change happened and how much was replaced. If you do not own a kit, take a sample of tank water to the shop before you change it, so they test the water the fish were actually swimming in when the signs appeared.
Log the tank's recent history too. New fish, plants, or decor added; a new food opened; a filter cleaned or media replaced; a heater fault; a missed week of maintenance; a child who may have fed the tank twice. Owners often record the symptom and forget the change that preceded it, yet for fish, the change is very frequently the clue.
Capture what words describe badly
Take clear photos and a short video of the symptom while the fish is in its own tank, in normal light, before you catch or move it. Flashing against decor, clamped fins, a strange swimming pattern, spots, and breathing rate are all things an expert can judge in seconds from footage and only guess at from a description. Photograph the whole tank as well as the fish: stocking, planting, equipment, and algae growth all carry diagnostic information, and a shop may spot an overstocked or under-filtered setup from one wide shot.
If the fish's condition changes, take a new photo in the same spot and light. A series showing a patch growing or fins receding over days is far more persuasive — and more useful — than either image alone.
Arrive with the packet complete
Before you go, assemble the timeline, the water readings, the tank history, the species and ages in the tank, what the fish are fed, and anything already tried, including advice followed from elsewhere. Say plainly what has been added to the water or given to the fish, even if you now suspect it was a mistake; vets and shops can work around an honest error far better than around a hidden one. If a fish has died, say when, and whether its body showed the same signs — that is diagnostic information, not an admission of failure.
Keeping that packet assembled in advance, rather than reconstructed in a shop doorway, is what KinsLe's Pet Passport is for, holding this fish's records and your diagnostic notes together in one portable packet for the vet or shop.