As usual my experiences are different. I was forced to carry my entire stock of fish sometimes when shifting. So I carried them in empty paint drums. I would cut of a length of polyethylene tube a little larger in diameter than the drum, and couple of feet longer, tie the bottom end and insert it into the drum, pour in water an inch less than the drum height, make an air-head with the rest of the tube. If the distance would be long then I would change the air-head some time, by opening and re-tying it. That way I could use more water and not worry about the bag bursting, or the fishes getting thrown at the walls.
You make the air-head by pulling the top portion of the poly-bag by pinching two ends this causes the two faces to come together; and then keeping the tension gather the entire top together and twist it and the air inside the bag remains there.