With the low speed specified (36 rpm), the best that can be done is to use two pitched-blade turbines (four blades each, at a 45 degree angle). Each pitched-blade turbine needs to be 600 mm in diameter with 150-mm wide blades. The tank should have four wall baffles to prevent swirling and to keep calcium carbonate from accumulating on the bottom. The drive motor needs to be at least 0.25 kW.