I´ve created a cube, cut the top surface to create a open tank, extrude the sides and the bottom and placed a flip tank inside. No I will let fall a cube in it, but it´s collide on the top of the tank as if it is still a closed cube.
Try checking “Output Back” on the polyextrude node. Fluids can't collide with a paper thin face. Fluids and gas require an actual volume to intersect with.
“Output Back” doesn´t help. The Problem is, thats the falling cube collide, not does it not do, but its collide on a not existing top (as if the tank is a full cube and the blast and extrude nodes would be ignored).
Change your Solver from Bullet to RBD. Bullet uses Convex shapes, so even though you cut out a side of your container, Bullet considers it a solid cube. RBD interprets it correctly.
