Get your aquarium to send you a text message alarm when your aquarium leaks/overflows!
This sensor only takes a few hours to build and you can use it to detect the presence of even small amounts of water on the floor. Connect it to a buzzer to provide an audible alarm, or if you own a Neptune Systems Apex controller, you can have it alert you with an email or text message.

How it works: The idea behind this sensor is to make a simple switch that closes when it comes into contact with water. Two wires in a speaker cable serve as the two sides of the switch. The cable is cut at regular intervals and reconnected with uninsulated butt-splice connectors. These connectors conduct electrical signals from the copper wires inside the speaker cable to the outside environment, and therefore each pair of connectors is a switch that "senses" the presence of water. When a puddle of water contacts two crimp connectors, current can now flow between the two wires of the speaker cable, closing the switch. This signal can be used either to turn on an electronic buzzer or be fed into your aquarium controller to do things such as switch on devices or send a message.

Pretty much everything necessary for this project other than the aquarium controller (if you choose to use one) can be bought at a hardware store.

Materials Needed:
     14-gauge speaker wire - Get as much as the intended length of the sensor. I chose a thicker wire because I wanted to minimize voltage loss across the 20 feet that I bought. If you plan on having a shorter sensor, you can use a thinner gauge.
     Uninsulated butt-splice wire connectors - Buy 2x the number of points where you want the sensor to check for water - I got 40.
     Wire clips - Buy one for every pair of connectors (20 in my case).If you're going to be laying these down on the floor, you should probably look for adhesive-backed clips.

Tools Needed:
     Knife - for cutting apart the two strands of speaker wire.
     Crimping tool
     Wire stripper - you can use the knife for this if you don't own a wire stripper.
     Measuring tape

Alarm Circuit:
     Electronic chime(such as Radioshack item 273-071), 9V battery and some way of connecting wires (either solder and soldering iron or insulated crimp connectors)
     Neptune Systems Aquacontroller Apex and I/O Breakout Box (breakout box is not necessary but convenient)