Blast, this is rough drawing of what I'm talking about.

The loop rises above the maximum water level and it alone, where it not for the unintended creation of a siphon, would stop all flow away from the tank through the pump. The addition of a vacuum-break, a tiny hole or valve they opens when the pressure goes negative, prevents the creation of the siphon. The loop and vacuum break work together to prevent all flow once the pump is turned off.

If you go with a small hole it only needs to be big enough to allow in air and to not clog. A 1/16", or even smaller, will usually work as long as the water is not too chunky.

Some people advocate inserting a thin copper wire into the vent hole. This is bent to keep it from being drawn in or blown out and extends a bit into the water flow so it wiggles and vibrates when the pump is pushing water past. The vibrating wire tends to keep the hole from clogging up from fine particulates or calcium.