If you're using it as a top off pump, you just want one that can handle the head pressure (the height it is pumping) and flow isn't all that important. I'd guess you're looking at something that can push a 12' head. Most aquarium pumps have a chart on the box (or at the very least on the manufacturer's website) that show you flow versus head. If it doesn't go up to 12', I'd say it's too small in your application.
It is a violation of federal law to use this product in a manner inconsistent with its labelling.