I am not a pro by any stretch but fish usually die from either temp, pressure differences, lack of O2, improper handling, stress, or any combination there of. Get them in the box quickly with as little handling as possible make sure aeration is up to snuff not just recirculation from lake surface water. Hot water holds less O2, holding tanks are best when they are round and not square. Keep your hands clean and if you partner is ugly or looks hungry have them look away...