I have what seem to be easy questions but I am sure is pretty complex. I understand that the lower power you have the lower the letter of the impeller AA-C I think I have seen. My questions are what makes an AA and AA and a C a C (Diameter/Angle of the Individual Blades/Quantity of Blades)? How do they function differently from one another (Pressure/Flow)? What changes need to be made the to rest of the pump to use each different size of impeller?