Preparing the fish before the bag
Stop feeding a full day before travel. A fed fish fouls its small volume of travelling water quickly, and most sick-fish journeys go wrong through ammonia in the bag rather than the journey itself. Catch the fish gently with a soft net or, better, herd it into a submerged jug so it never leaves water and its slime coat and any sore skin stay intact. If the fish is being taken because of a suspected water problem at home, also fill a separate clean bottle with tank water for testing — the adviser will want both.
Bagging and boxing for the trip
Use a proper fish bag, ideally doubled: fill the inner bag about a third with the fish's own tank water, add the fish, and fill the rest with air before tying it off, rounding or taping the corners so a weak fish cannot wedge itself into a fold. Place the bag upright in a dark, insulated box — a cool box or a box lined with a towel — packed so it cannot roll, and keep the whole box level in the car rather than in a hot boot or a cold footwell. Darkness is not a nicety; it is what keeps a sick fish still and conserving energy instead of panicking against the plastic.
On the journey itself
Go directly, and keep the trip as short and smooth as the route allows. Keep the car at a steady, comfortable temperature — tropical fish lose heat quickly in an air-conditioned car pointed at the box, and a parked car in sun overheats a small volume of water fast. Never open the bag to check the fish en route; every opening loses heat and oxygen and adds stress. If the journey is long, tell the shop or vet when you book, because they may suggest a larger water volume or battery-powered aeration for the trip.
Handing over useful information
The person advising you can only work with what you bring. Be ready with how long the fish has been unwell, what changed in the tank beforehand — new fish, a filter clean, a missed water change — what the home test readings were, and anything already added to the water by way of treatment. Arriving with guesses wastes the appointment; arriving with the tank's recent story usually gets to an answer in one visit.
Getting the fish home again
Float the closed bag in the tank briefly to equalise temperature, then release the fish gently into dim light and leave it alone to settle rather than feeding immediately. Test the tank water that evening and the next day, since a tank that made one fish sick has a cause still in it until proven otherwise. Keeping the fish's symptoms, the advice you were given, and the home readings together in its Pet Passport on KinsLe means the whole episode is on record if the problem ever returns.