tank overflow prevention?

[cwickham]

hey

i use ball-valves to limit the return from my main tank to the sump. I adjust the level of the tank so that the return is completely under water to reduce micro bubbles and noise.

The problem is that if the power goes out and the tank level drops, air gets into those pipes and when the power comes back on, the air gets trapped at the ball-valve and limits the flow even more cause my tank to overflow if im not there to release the air.

is there any way around this, is there somthing that will shut my return pump off when the water gets to a certain level(just for emergency if im not home)

i have tryed to find a normally open float switch to make somthing, but can only find normally closed ones.

thanks for your help
craig