I had a well that wouldn't drain so I was investigating the problem, in the process I couldn't believe just how nasty my livewells had gotten. I use a SureLife product and Hydrogen Peroxide when the weather (Heat) calls for it like many others do. Obviosly it had created a thick mildewy type coating all over both tanks, come to find out it's very UN healthy for the fish being held in there, sometimes for extended periods..
You can power wash them like I did, or go to the car wash and hose them down good. Mine look like new again
I also blew the drain hole out to find a 4" swimbait clogging the drain line laying on the ground, I guess one of my fish had spit it up and I never saw it before opening the drain...
Just an FYI, on helping keep our fish and livewells in good shape..