Is this stuff aquarium stuff if the tank is left empty a week or so
while the stuff dries and outgasses?

http://www.tapplastics.com/shop/product.php?pid=132&

I have a tank I want to reenforce by gluing some 1" square rods in
all the corners. I need something that will fill in the gaps between
the rods and the tank since the rods aren't perfectly square/flat.

ADVthaAnksNCE