Be sure you have a pump that has enough head pressure to push water up to the top of your container.

I have a 55 gal drum and first tried to use a small Rio pump that I had as a spare. When the water level got to about 5 gallons, the pump would only push water to the top of the barrel, but not over the top.

I use a Maxi-Jet with a 1/2 tube on it. Drop it in the sump to empty it, then drop it in the water barrel to refill the sump.