Totally unsafe unless absolutely sealed off from tank water.
There's the rub. It can
be used in framework but it has to be sealed away, and then it doesn't last as well as other materials can. There is a synthetic used in work boat construction that when sandwiched between layers of fiberglass material ends up stronger and considerably lighter than plywood of the same thickness. Plus, it won't rot. I helped a friend build the wheelhouse for his crab/lobster boat that was 8'x10'x6-1/2' that when it was completed, before we glassed it to the boat deck, the two us could pick up with two or three fingers on each hand without it flexing
. The whole wheelhouse weighed maybe 60-65 pounds. Amazing stuff. Handles like styrofoam pretty much until you give it the glass/fabric sandwich treatment. I gotta ask Hector what that stuff was called, because I can't remember.