I replaced a leaking tank in my 20' skiff a few years ago. I researched a bit and found that an epoxy coated aluminum tank is the way to go. I placed 3 butyl rubber strips on the floor of the tank pocket running fore and aft with gaps between them. This keeps the tank up off the floor and lets water get out. You don't want to allow any standing water against the tank. I used a 2 part polyurethane foam from Aeromarine to lock the tank in position. Make sure your installers pay attention to details. This will make a big difference in the lifetime of the tank.