How does anyone know if dropping them in ice cold water is extremely painful? I'm not saying I advocate the practise or not but how can you be so sure that it is "extremely painful"?
I certainly cannot answer the question but I ASSUME they watch behavior to see what reaction occurs. I also have an observation.
I buy frozen shrimp, and frozen blood worms. I plop the iced cube into the tank whole. Admittedly the tank water is warm.
The fish pick at it as it melts, often grabbing it and running away (it's funny as often it is a pretty small fish towing this relative ice berg).
Are they so interested in food they endure severe pain?
Or is it not that painful?
Maybe I'm cruel for feeding them thus; I never really thought about it until this conversation. But I never seen any reluctance to grab onto it from any of several species of fish. And as soon as they eat whatever they pulled off, they make another pass.
Again - it's in warm water, it is perhaps a different situation. The only other thing I can draw from is my own experience. If I swish my hand around in very cold water with melting ice cubes, I see very little difference in sensation whether I am in just the cold water, or touching the ice cube.