Make sure to get the corner blocks!
That is, the blocks where the walls meet. You can't see them directly without removing the walls, but they look like...
L
L
L
GLLL
If the L's are two intersecting walls, then G is the portion of the wall that counts as part of the tank that you cannot see from the inside of it. Make sure to check and see that it's not made of a material that cannot be part of a xycraft tank, such as dirt, or some ores.
You'll find corner blocks like that where the walls intersect each other, the floor, and the ceiling. Check them all. It's easy to simply replace the surface blocks without replacing the corner blocks as well with underground tanks.