Does Anyone Know How To Make A Pump Pull Water From Tank Below?

[fishwarrior]

I have my sump tank on top and the display tank below.  I thought if I put the pump inside the top tank and pull water from the bottom tank, it would act like an HOB filter.  The pump is not pulling the water.  I'm using a mag drive pump and also added water in the pump and in the pvc pipes connected to the pump, just don't know why it's not pulling or am I missing something?
 
Are pumps not able to pull from the bottom?  I know they are made to push water up, but why can't it pull water up?

 

[DHJac]

Most aquarium pumps have limited suction head capabilities, which is the distance from water level you are drawing from to the pump impeller.  Have you checked the capabilities of your model?  I know they list the discharge head the pump is rated for on the box for most aquarium pumps (usually 4-6 ft) but I don't know if they give the suction head as well.  If you are trying to pull more than a couple of feet, I doubt it can do it.  You could add a second pump in the suction line that acts as a booster.