Hydrostatic pressure HELP!

Hi, I have a question (which may seem very simple) but it is really bothering me. If we have a body (let's say tank) partially submerged in the liquid (and it is also filled with the liquid) why pressure at the bottom face is acting upward ( it's usually drawn outside the tank, like it's pressure due to outside fluid)? I understand there's atmospheric pressure acting on the top face, but why pressure at the bottom of the tank is not equal to atmospheric pressure+ pressure due to the fluid inside the tank (density*g*depth) but DOWNWARDS as well? Also, in this type of problems, do we consider pressure due to surrounding fluid ( e. g water in which tank is submerged , which would make sense as pressure would be upward at lower face). I would really appreciate a clear explanation as I really confused myself by thinking too much about it. Thanks in advance