Couple o' things . . . installing a ball-valve between the pump and tank will allow you to throttle the flow down a bit.
Also, in my AGA tank w/overflow and MegaFlow sump changes in water level due to things like evaporation or adding/subracting larger things (like rock) are reflected in the sump, not the tank itself. Thus, I have to check the water level in the sump to determine if/when I need to add some water.
You didn't mention exactly how fast your sump was emptying (Although it sounds pretty fast), but remember it's not uncommon to lose a gallon or so a day to evaporation and if I don't add some water every two or three days, my sump level would soon be down below the pump's intake.
Ideally, a pump should return water to the tank, factoring in headloss, close to but not exceeding what your overflow can drain. Pumps lose pressure and have a lower GPH if they have to pump to a height (Like from a sump to the top of the tank) called "headloss" with "Head" being the height from pump to outlet. Most sites selling pumps, the pump manufacturer's site, as well as the operation manual included with pumps have flow-charts that show what the pump is putting out at a certain head. Calculating your "head" and checking the chart will show you what a particular pump is giving you.