There's a common theme going around, that "breathable" shipping bags are better for shipping fish. We disagree and here's why:
First, all plastic bags "breathe". It's just a matter of degree. While some plastic films are more porous than others, the typical style bag used for shipping over the past several decades transfers lots of oxygen through its membrane. This is why we recommend never floating bags during the acclimation process. Floating surrounds the bag in oxygen-poor water rather than oxygen-rich air. In addition, the oxygen in the water is dissolved and not available to transfer through the bag.
You have probably heard the standard guideline that the older style bags should be 1/3 water and 2/3 air. This is a fallacy which we have disproved a few hundred-thousand times over decades of shipping. Through extensive testing, we've found that there is actually greater oxygen transfer to the bag-water when it is 2/3 water with only 1/3 air. Many are convinced of the former, so here's the theory behind our assertion. It's very simple: The surface area of the water in contact with air inside the bag is exactly the same at 1/3 water as it is at 2/3 water. Equal surface area means equal rates of exchange of oxygen between the water and air. Air holds far more oxygen than water--especially when pure oxygen is pumped into the bag. Having 1/3 pure oxygen in the bag provides more than enough oxygen. More water in the bag means greater surface area of the oxygen-absorbing water with the external environment. Add in the fact that more water gives a greater buffer against ammonia and carbon dioxide build-up, and you can see why two thirds full is better than one third. A breather bag will only contain slightly more water than a properly prepared regular bag.
Now, here's the big reason why "breather" bags are not the best choice. They are thin and if double-bagged, then transfer is cut down. There is more risk of puncture. Also, if using a heat pack in the box, the heat pack depends on oxygen to function. A heat pack can quickly deplete the oxygen in a box, leaving nothing to diffuse into the bag, thus suffocating the fish. By pumping the bag with oxygen, you prevent the heat pack from taking needed oxygen from the fish. We find this absolutely essential in cold-weather shipping.
In summary, breather bags cannot provide pure oxygen, so less is available during a shipment, they are thin and not as effective when double bagged and can run out of oxygen in the presence of heat packs. We could easily choose to use them if we wanted, but after shipping many hundreds of thousands of fish, we've concluded that the "breather" bags are not the best choice for shipping most fish.
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