The short answer
Transport fish in bags (or a lidded container) of their own tank water, kept dark and warm. Fill the bag about one-third with water and two-thirds with air, keep the trip as short as possible, and settle the fish back in slowly at the other end. The two things that matter most are stable temperature and not overcrowding the container.
Bag them properly
Use the fish’s existing tank water, not fresh tap water — it’s already the right chemistry and temperature, so there’s no shock. Fill the bag roughly one-third water to two-thirds air so there’s plenty of oxygen above the surface, and only put one or two fish per bag; crowding fouls the water fast. Double-bag to guard against leaks, and don’t feed the fish for a day beforehand so they produce less waste in transit.
Keep it dark and warm
Fish are far calmer in the dark, so wrap the bags or pack them into a closed cooler box or polystyrene box. That box does double duty as insulation, holding the temperature steady — the single biggest risk on a journey is the water getting too hot or too cold. In winter add a wrapped warm (not hot) pack; in summer keep the box out of direct sun. Aim to keep the trip short; fish can stay bagged for a while, but the sooner they’re home the better. See how long fish survive in a bag.
Settle them back in
Treat arrival like adding new fish. Float the sealed bag in the destination tank for 15–20 minutes to match temperature, then acclimate gradually before releasing the fish — net them across rather than pouring bag water into the tank. Keep the lights off for a few hours so they can calm down. Our step-by-step acclimation guide walks through it, and if this is a new setup, check the tank is stable first with a test kit.