When plastic bagging fish for transport, use only enough water to just cover the dorsal fin. Ensure the bag is of sufficient size that the fish can have an inch or more space from the sides and ends of the bag (and sufficient strength to support the weight of the water and fish. 3-4 mil thick bags are normally used). Squeeze out the current air, add 5-10 times the amount of oxygen as water. This is normally sufficient oxygen for up to 24 hours. If oxygen is not available, just the air in the bag is sufficient for an hour or so.
Ammonia build up and temperature control then become the major problems. Placing the bags in a Styrofoam picnic cooler can help maintain temperature controls. Cover the top of the box that the bags are in. The darkness soothes the fish a bit and thus decreases the ammonia generation. If transporting in a car, put the cooler in the passenger compartment, not the trunk. This is of more importance during the summer than winter but applies to all times. Do not leave them in a locked car while stopping for lunch, use a fast food drive through.
Based on reported controlled experiments, it was found that floating transport bags in the pond for 30 minutes prior to release slightly decreased the mortality rate, particularly for small fish. This test was conducted with the fish bagged for one hour. For fish that had been bagged for four hours, it was found that the mortality rate increased slightly for all sizes of fish if the bag was floated for 30 minutes. My recommendation is that if the fish have been bagged for four or more hours, and the temperature difference between the transport water and pond water is less than 10 degrees, it is better to release them immediately than to subject the fish to the "bad" water in the bag for an additional half-hour. Thirty minutes of floating will prevent some shock if the temperature difference is large, but it will not acclimatize the fish to the new temperature. Actual temperature acclimation of a fish takes several days, similar to us dealing with jet lag. It is not only the temperature the fish needs to be accustomed to but also the pH, hardness, alkalinity, "the taste", etc. of it's new surroundings.
Remember the relationship between pH and ammonia toxicity? When the fish are in their sealed transport bags and expelling carbon dioxide, it causes the pH to drop. This makes the ammonia less toxic. I have made measurements of fish bagged for over 24 hours and found pH readings below 5.0 with ammonia concentrations over 12 ppm. Yet the fish were in very good condition. When floating a bag, do not open the bag until ready to release the fish and DO NOT MIX ANY POND WATER WITH THE TRANSPORT WATER. Just opening the bag can release some of the built up carbon dioxide thus raising the pH, and even worse, mixing pond water into the transport water can suddenly raise the pH to where the ammonia is highly toxic. Bottom line is never add pond water to transport water and never add transport water to pond water.
If a transport tank is being used for moving fish, an air stone or aeration column can be used. A venturi (air jet) is not recommended since the strong currents induced require the fish to "work" harder which increases both the oxygen consumption and, of more importance, the ammonia waste products in the small tank. An air stone can be fed directly from bottled oxygen or from a small air pump. An aeration column can be fed from a small submersible water pump ideally located at the opposite corner or end from the aeration column. Fish can be transported more safely in a sealed, oxygenated bag than in a transport tank.
CAUTION: Make sure that the transport tank's air supply cannot be contaminated with the vehicle's exhaust. Carbon Monoxide is very soluble in water and can be even more deadly to the fish than to you.