This is what i have it's the best!!!
Auto Top Off Controller
* Automatically refills evaporated aquarium water
* Maintains aquarium and sump water level to prevent pump damage
* Save time and effort by automating aquarium top-offs
Automatically refill your sump or aquarium without lifting heavy buckets or spilling water. Floating sensors monitor water level and activate pump when water level drops due to evaporation. Helps maintain stable salinity/specific gravity and optimum water level to prevent pump from running dry. Features LED panel, Audible Signal alert and linkable brackets for installation. Requires water pump, sold separately.
Pump Safety Protection: The A.T.O. Controller automatically stops power to your top off pump in the event that your reservoir is low on water. The safety feature prevents pumps from running dry and failing.