Temperature First, Lights Low
Whatever the journey, acclimation starts the same way: with the bag still sealed, float it in the destination tank so the water inside slowly matches the tank's temperature — fifteen to twenty minutes is typical for a small bag, longer for a large one. Keep the tank lights off or low and the room quiet, because a fish that has travelled in darkness is dazzled and stressed by sudden brightness, and calm now pays off in appetite later. Have everything ready before you open anything: a soft net, a clean jug or bucket for bag water, a towel, and ideally a quarantine tank prepared, since travelled fish are exactly the fish most worth observing separately before they join your stock. Which method comes next depends entirely on how long the fish has been sealed in.
Short Trip: The Gentle Mix
For a fish that has been bagged for a few hours — a drive home from a shop, a move across town — the water in the bag is still reasonably clean, and the kindest route is gradual. Once temperatures match, open the bag, roll its top down so it floats upright, and add tank water a small cupful at a time every few minutes for half an hour or so, until the bag holds mostly tank water. This walks the fish slowly across any difference in chemistry between the shop's water and yours. Then net the fish out and release it; the bag water, with whatever the journey stirred into it, goes down the sink, never into your tank.
Long Shipment: Speed Beats Drip
A fish that has spent a day or more in a shipping bag is a different case, and the gentle method becomes the dangerous one. Inside a sealed bag, the fish's waste builds up while dissolved gas from its breathing keeps that waste in a less harmful form; the moment you open the bag and the gas escapes, the chemistry flips and the same waste turns sharply toxic. So for posted fish: float for temperature with the bag sealed, then open it, net the fish promptly and move it straight into clean tank or quarantine water — no leisurely drip in water that is now working against it. The fish may look pale and sit quietly at first; that is the journey, not necessarily injury, and most recover their colour within hours in clean, calm water.
The First Day in the New Tank
However it arrived, give the fish a soft landing. Keep lights low for the rest of the day, hold off feeding or offer only a tiny meal late in the day if the fish is actively looking, and resist the urge to keep netting, moving or rearranging around it. Watch breathing and posture from a distance: steady gill movement, an upright body and gradual exploration are what you want; gasping, lying on its side, or white patches appearing over the first days are reasons to test the water and seek advice from a veterinarian who treats fish or the seller who shipped it. A quiet first day, in water that matches what the fish will live in, undoes most of what travel did.
Help While Your Hands Are Wet
Acclimation choices hinge on how long the fish was sealed in, and it is easy to second-guess the drip-versus-move decision once the bag is open and the fish is waiting. Settle the fish first, then revisit the reasoning while the details of the journey are still clear in your mind. Put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.