I've also had a bad experience with an over flow but it was due to an experiment I wanted to test out but back fired. It did however lead to another idea as a preventative measure. I rigged a float valve in a pill bottle to the rim of my display tank and wired it up to a smoke detector. If the water begins to rise to a certain height the smoke detector will sound off. I had to modify the pizeo so I won't panic thinking the house was on fire. It'll still sound off if it detects smoke around my tank. It has gone off when I've added to many bags to acclimate water temp but shuts off when the overflow is in the sump.