I bought a used bass boat and when purchased there was some mold on the sides of the livewell. What do you use to clean out livewells without killing fish later on?

Try this stuff...  BoatLife Bilge Cleaner

Link is below

http://www.jamestowndistributors.com/userportal/show_product.do?pid=8967&familyName=BoatLIFE+Bilge+Cleaner

To clean your livewell, you can use the same BoatLife Bilge Cleaner - it's non-toxic and biodegradable.

The product can be wiped sprayed onto the surface to be treated. After a few minutes, rinse with fresh water and whisk away fish blood, slime, mold, mildew stains, rust stains and water deposits. It is safe for use on metal, fiberglass, rubber, plastic and painted surfaces. Used as directed, there is no residue left to contaminate fish or damage aerators and pumps.  Make sure you get to the sides of the lids where you can't see - it's a prime spot for mildew to start.  

Boatlife Bilge Cleaner emulsifies oil and scum to clean bilge areas and live wells. It's non-foaming, biodegradable formula contains no phosphates and leaves behind a pleasant fragrance. Tip: Did you know that Boatlife bilge cleaner also makes an effective cleaner for carpet, upholstery, fabrics, concrete? Use it to remove mold stains and mildew stains.

One of our sponsors sells some stuff called TLC.

http://www.sure-life.com/pro_TLC.html