anyone know a easy way to make return pump on a flat switch?

im building a set up with low level tank that sits directly under flood and drain tables , the tables are fed via a large upright flexi tank, when the tables drain into the tank below i need a pump with some kind of float switch to kick in to pump the solution back to the upright tank. the low level tank is not big enough to hold enough water to be able to flood both tables and i prefer having the res outside of the grow room.

i want the pump to kick in as soon as there is a few inches of water . any ideas and where i can find the parts to make one ? don't really want to use a sub pump as these can get air locked and are not designed to pump low level water away