As is advised with most fish you should avoid sudden changes in water temperature (for example by introducing new water that hasn't been left to come to room temperature). I have, however, never understood why that is the case, in particular given that most fish are cold blooded so there is no issue with needing to "ramp up" the metabolism when in cold water.
Say a particular species of fish can survive in water between 20°C and 30°C what causes a fish that was previously in 30°C water to die when suddenly plunged into 20°C water? I understand this effect is called "shock" but I'm looking for the biological reason for it.
To be clear I'm not looking for "chemical x is released and that leads to the fish dying, I'm looking for why these chemicals are released at all. So to give an example if my question had of been "why does adrenaline shock kill" I'd be looking for "adrenaline release is usually a good thing, it's released in response to a fright and prepares the body for a fight or flight response. However this heightend preparedness can cause additional strain on the body which can cause death in some cases"