There is no one answer to how long a fish can survive in a transport bag. Under shipping conditions, a fish can survive up to a week. However, the trip home from the pet shop often does not feature the same protections. When you leave a pet shop with a new fish, go straight home; the fish is likely to survive only a few hours under ideal conditions.
Different rules apply when bringing a fish home and shipping fish. Properly packed, a fish can survive for up to a week in a bag for shipping. However, never keep a fish en route home from the pet shop in a bag for more than four hours in ideal conditions. Avoiding temperature extremes helps the fish survive, but if you must purchase one on an overly hot or cold day, put the transport bag in an empty cooler for the trip home. Make fish shopping the last errand of the day; cars are not a good place to leave fish.
If you plan on shipping your fish, you will need certain supplies. Polystyrene foam-lined shipping box are hard to find, but carefully cutting pieces of polystyrene foam to shape to line an existing box creates the same effect. Many hardware stores carry polystyrene foam sheets. Some online retailers and local fish stores carry plastic shipping bags for fish, as well as rubber bands, stress coat -- a water conditioner -- and a way to inflate the bag. A regular aquarium air pump with a short length of airline tubing will work in a pinch, though you may want to get a pressurized bottle of oxygen when shipping a fish for more than a few days. Just hold the bag closed around the tubing from your pump or oxygen bottle, and pinch it closed before rubber banding it. During cold weather, get hand warmers from sports stores to keep the box warm and cold packs for warm weather.
Before bagging fish for shipping, add a dose of stress coat to the water. Then, add a little water to the bag. You want more air than water in the bag; during the trip, most of the oxygen will come from the air in the bag rather than the water. You want no more than 25 percent of the bag to hold water; ideally, just enough to completely cover the fish from any angle. Use a larger bag if necessary.
Preparing for Shipping
To ship the fish, hold the neck of the bag closed around a length of airline tubing. Using either an air pump or an oxygen canister, inflate the bag without over-inflating it. Then, rubber-band the bag closed near the top. Use a second rubber band to make sure it remains closed. Rubber band the bag's corners as well to make it harder for the fish to get stuck in transport. Place a second bag upside down over the first bag, then rubber band the top. This will make it less likely to leak. Secure the bag within the shipping box with standard packing materials such as newspaper to keep it in place. If you use a hand warmer to heat the box, duct tape it to the lid of the box, covered in damp newspaper. Then, seal your package.
Check with your postal carrier before shipping. In the U.S., the post office accepts live fish, but sometimes individual clerks don't know this. Always check with the office beforehand, since you may need to speak to a manager before they'll accept the package. Also, the manager may know the best time to drop the fish off for pickup so it will spend less time in transit. Write "Live Fish" on the outside of the box to alert postal employees that the box contains live animals.