Homedepot sells Desiccant packs. this stuff will pull moisture out of
the air and into a drip container. The pain is making sure it has balls
all the time and the cost to keep refilling over time, like 4$ for a
bag. If you cover your sump more than likely you will raise the temp of
your tank. I say take the tank out and use boat varnish on the
interior. 12$ at Homedepot.