Thanks guys for the clarification.
I was asking about the temperature shock change to the fish. I knew there had to be a simple solution. I didn't know the haul trucks could suction from your BOW to slowly bring the fish tanks to the same temperature as the pond.
They either do that water exchange at the pond to the tanks on the truck (I don't like to do that because the added risk of bring back nasties to the farm which means more labor sterilizing everything) OR have a good idea of the water temp in the ponds in the area where they will be delivering the fish that day and do the tempering before the truck leaves to deliver the fish.
Depending on the fish, 3°F difference in water temp is safe. You might be able to get away with a 5°F difference with larger fish. Some fish are more sensitive than others. For instance Golden Shiners are more sensitive than Fathead Minnows.
For us here in Northern Indiana, water temp has to be in the mid 60's AND after Mothers Day weekend for us to stock Tilapia.
If the customer wants them stocked earlier, we try to discourage them, but if they *have to* have them we tell them if the water cools and they die, we will NOT replace them on our dime - they will have to re-purchase the fish. Lets just say that you learn some things from experience.......
It's not only temp, fish don't like large pH swings either.....
