Hey Puff,
I have a small hold drilled just above the water line on my hose coming out of my sump. That way, if you lose power, the hose does not become a reverse siphon and empty the tank back into the sump (which would eventually overflow). As soon as the pump stops (eg.,power outage), air travels from the drilled hole, breaking the siphon. Water continues to flow out of my overflow, until that level is reached, and the cycle comes to a halt.
A little bigger than a pinhead hole works great for my scenario, like about the size of a 4 penny nail.
When I brought up the tank, it was absolutely necessary to try this out, because I live in a condo. The lady beneath me wouldn't enjoy dripping ocean coming from her ceiling. 8O
HTH,
~wings~