Originally Posted by msjinkzd
because as ammonia is produced through waste, it drives teh pH down, rendering the ammonia non-toxic. There is no exchange of gas. When you allow the exchange of gas, it allows the pH to stay stable (in my case moderately hard) and allows teh ammonia to become exponentially more toxic.
This is why "breather bag" shippers add a drop of Seachem Prime to the shipping water. This solves the problem of ammonia toxicity. I've had fish last a week in transit in their breather bags with no problems. In my opinion, the biggest drawback to breather bags is the expense.
If your water is quite hard, it is well buffered from pH swings and may not change that much, even if there is no gas exchange in your poly bags.
If there is a substantial pH change during shipment, as you suspect, then pH "bounce" is probably something to worry about during acclimation to the new tank.