I use bleach water and recirc it to kill anything that might be alive, I fish Asian carp rivers so I keep my boxes clean. I was hesitant about bleach but after a good flush you should be good. Plus many livewell treatments like Please Release Me have additives to counteract chlorine. That's also helpful for the summer months. You can mix the treatment with tap water and freeze it empty bottles, then toss them as needed to cool the fish