Its no surprise to me that the tank isn't empty at t=120 min but only a bit later. In case of the values in your sheet the tank is empty after 157,083 minutes, then the water height in the tank gets negative(!).
Simply let the solve block to solve until t=160 minutes to see it.
I also deleted the solve block for V, it wasn't necesary at all as V is simply calculated directly. The values of V are imaginary but as they are used later only squared, they yield a negative real result. Not sure if thats what you intended, though, and you might want to check the formula for V.
Is also noticeable that the water height in the tank is negative for the first five minutes (see attached sheet) which clearly isn't realistic, so there seems to be still something wrong with the model.Maybe it has something to do with the negaitive V^2?