In bag with pure oxygen is best. Next best ice cooler with airstone. Either way you should put something in the water to bind the ammonia.If you do have a buildup of ammonia the PH in the water will lower making it less toxic UNTIL someone drips new water in to even out the PH then it does become toxic .Do not do a drip without ammonia binder. I prefer to just move fish from the bag to the tank without a drip and never had "PH shock".