or when wood has to touch concrete.
I don't know about down there, but up here when we have wood on concrete we seperate the two with a strip of polyethylene foam. Which is what I would use in this case if the floor is damp. If the floor has standing water, then I would use cement blocks and polyethylene foam.
The link is for a sill gasket but I have used this material between the foundation and the walls of a two story house before so don't fret yourself over the weight of your fishtank crushing it and causing it to fail. It won't.