OpenGL C++ rotating objects around the center of scene

I'm making the fish in aquarium and I want to rotate the "walls" and bottom of aquarium around the center of aquarium with dragging the mouse. Except that the scene a bit rotated to be "isometric". The walls just flying like they want (around center, but affecting the shape and position of walls causing them to separate). Here is how it looksI'm making the fish in aquarium and I want to r