I add a frozen water bottle the night before just before I go to bed so the bait will be cool to start the day. I have an Engel bait cooler which helps with the heat while fishing. I try not to use anymore frozen bottles but I will add lake water first thing, just a little bit to level things out. Just my .02.
papasage LIKED above post
What several people have said is spot on. It is a matter of slowly acclimating them to lake temp while fishing. I keep mine at about 55 degrees in my minnow tank at the store. That maximizes survival. If your goal is keeping them for an extended time between trips you need 3 things - cool water - fresh water - oxygen.
Specklocker LIKED above post