I use the 4 inch AC fan from radioshack.
http://www.radioshack.com/product.as...5Fid=273%2D241
It is very silent and doesn't draw power from the power supply. I just wire it with the pump. The blades and rotor are made of steel and are heavy. It takes a little while to get up to speed, but once there, remains there. It is silent and sucks through a heatercore very well.