NEVER try to regulate the water draining to the sump. Any and all attempts will result in failure.
Sumps fill through gravity draining (not siphon) from overflows in/on the tank. Either hang-on or bulkhead overflows are used. The level of the overflow determines the water level in the tank. Once the tank level reaches the bottom limit of the overflow, water flow ceases. A pump returns water from the sump to the tank. This 'filling' action of the return pump is what perpetuates the spill-over in the overflow's gravity drain.
Since the tank maintains a constant level, water loss (from evaporation) can only be observed in the sump.
A limiting factor on the size of the pump is the size of the overflow drain. Single 3/4" drains have a flow restricted to 600gph or less. So, choose the size of your drain by the rate of the pump needed (at head). You're looking for anywhere from 5-10 full tank cycles per hour. At head is the amount of water flow at the point where the water actually enters the tank. An example would be a pump rated at 1200gph would have a drop in flow of 400gph at a 5ft head (pump-in-sump to tank). So, the actual water circulation of the system is 800gph due to the head loss of that pump.
