The only fish that I have had die in a livewell are fish that are hooked bad. Would they die if you turned them loose? Of course they would. Anytime you stick a piece of metal in an animal it has a chance to die. Just take care of your fish the best you can and most will be fine. The ice bottle technique works really well for me and keep the recirc on constantly. Another little trick I do is when it is really hot I will throw a bag of ice in the livewell before I leave home and this cools down the livewell before I even get to the lake.