i warm my rodents up the same way and have noticed that if i leave them in the water for too long then they get a little soggy and start bleeding from the nose.

maybe cut down warm up time and see if it helps. if youre taking them straight out of the freezer and placing them into the water then maybe first try thawing them for an hour or two and then warming them in the water for only 15-30 mins. it may help...