Hi Houliham, welcome to the forum....

Badflash and I, must be reading your question differently...?

A good percentage of systems are CHIFT PIST ...constant height in fish tank, pump in sump tank.
The fish tank is highest, the growbeds at a lower level, the sump being the lowest point.
The water is actually 'overflowing' into the growbeds. The water drains from the beds to the sump. The water is then pumped back to the fish tank.

If the power goes out....the water just stops moving.
As long as your re-turn line doesn't start a siphon...use a check valve or simply, dont have it underwater in the fish tank....let the water spray into the tank to help with the O2.

The 'slo-drain' stands for 'solids lifting overflow'....the solids being fish pooh...
Hope this helps.