A modular tank comprises a plurality of panels positioned on the ground or another surface to form sides of a containment wall. Each panel comprises: i) a side wall section having a trapezoidal shape; and ii) a frame assembly connected to the side wall section and configured to support the side wall section such that a first base of the trapezoid shape is disposed proximate a base of the containment wall and a second base of the trapezoid shape is supported in an elevated position to form part of an upper perimeter of the containment wall. The modular tank also includes a liner configured to be disposed on at least one of the containment wall and the ground surface to thereby form a barrier for containing fluid in the modular tank. Depending on whether the first base of a panel is wider or narrower than the second base, panels may be aligned to form straight walls or curved walls, thereby enabling the modular tank to assume different shapes.