Making sure your fish tank does not overflow is important for your fish, your carpets, and possibly the people who live on the floor below you. Aquarium web site Monsterminibow details how to to make a DIY overflow sensor.
If you have a large aquarium, there's a lot of potential for water spillage if the drainage hole gets blocked, which can lead to floor damage and the loss of some expensive fish. The overflow sensor turns off power to the pump if there's a problem detected, ensuring you don't return home to a saturated floor. It's not a project for electronics newbies, but definitely an idea to consider if you've got a serious aquarium happening.