I know that down South chemicals have been used at tournaments. I am not sure what chemicals they are though.
I have found the key to be water temp.
I never fill my livewell up very long before I add the fish in preperation. I get the first fish and put it into the livewell while adding the water. In hot summer temps, the livewell water can become much warmer than even the surface water. Seems to me, that the warmer water holds less oxygen and for a shorter time. You do not want to shock the fish with a big temp change. In a lake situation, many times in summer these fish are comming from deep and much colder water than you fill up your livewell with from the surface.
Insulating your livewell is a good idea, this helps to maintain the water temp longer. Setting a timer to pump water regularly or pumping all the time can help too.
If you are keping fish to eat, forget the livewell and go right on the ice to a cooler. These fish are best to eat, that have been on ice and went in kicking!
Hawger