I'm looking for ideas on how to build a fairly low cost yet reliable water level sensor in an aquarium. In principle the sensor would be used in a greater aquarium system that controls numerous aquarium functions in addition to automatically changing aquarium water.
The simplest idea I can arrive at is a 'short' sensor, in other words, if the water is at the level of the sensor the circuit could be completed with a very small current thus signifying a tank full state, rather than a tank not full state. In practice the controller would use the sensor state to determine to open or close a solenoid that would remain open till the tank was full.
The application in mind is freshwater (with appreciable TDS to conduct) although saltwater applications aren't out of the question. Traditionally inexpensive float sensors are utilized (http://www.madisonco.com/level/HVAC.htm). The same manufacturer offers a conductivity switch (http://www.madisonco.com/level/M3850-conductivity.htm) that is both relatively costly and requires a costly controller to operate.
1. Low current as to not disturb tank inhabitants. They'd have to stick their head in it to get a surprise but I don't think it would take much to stun/kill a fish
2. Intermittent monitoring. As above to minimize disturbance.
3. Redundancy, failure in the sensor would result in water spilling onto a residential floor. Redundancy is necessary.
4. Provide a simple open or closed signal that could be utilized by a controller. Example: http://www.neptunesys.com/aquaControllerJr.htm. This model has the capacity to utilize a sensor to trigger a process.
5. Be as unobtrusive within the tank as possible, this is the disadvantage of the existing float sensors.
Can anybody suggest a starting point?