Water sensor for fish tanks

This is a water sensor I made for a friend of mine. You can hang it in a corner of a fish tank and it goes of when the water has reached a certain level.
It is designed to alert at 6cm or 3cm below the edge.

You will need an small switch (I got mine from an old printer), a 9V clip, a small piece of perfboard, a piezo speaker and a ping pong ball.