I'll take this from the top.
I am setting up a saltwater aquarium. The water in which the fish live must have a certain concentration of “dissolved organic content”. With too little “dissolved organic content”, nutritional requirements of plants will not be met. With too much, the health of other organisms within the tank will be compromised. There is a very broad range of tolerance that is acceptable to the inhabitants of the aquarium, both plant and animal. However, there is a range of concentrations of “dissolved organic content” that is optimal. A very common method, if not the most common method of ensuring that concentrations of “dissolved organic content” do not become too high is to route water from the aquarium to a "protein skimmer"
*.
Water within the aquarium is aerated. With greater concentrations of “dissolved organic content”, bubbles formed at the surface as a result of aeration will linger longer. All else being equal, a longer length of time for which a bubble exists on the water surface correlates positively to a greater concentration of “dissolved organic content” within the water.
From past experience, I know that bubbles form on the surface of the water at only one end of the aquarium, where there is surface disturbance created by an aeration device. Only under conditions in which the “dissolved organic content” is too high do the bubbles travel across the surface of the water to the far side of the aquarium.
I would like to have an electrode positioned under the surface of the water at the far side of the aquarium (away from the aeration device). I would like to have a second electrode positioned about 2 cm above the surface of the water nearby (also at the far side of the aquarium). When a bubble drifts into contact with the electrode positioned above the surface of the water, a path will be formed through which a current can travel from one electrode to the other. The current path will be from one electrode, across the surface of a bubble, through the air/water interface, through the water in the aquarium, to the other electrode.
When current travels from one electrode to another, a circuit is closed, and a pump must turn on. The pump will move water from the main tank of the aquarium into a protein skimmer. The pump is powered by AC current from wall mains. Therefore, the pump will be on when the AC jack into which it’s plugged is energized. I want to control whether the AC jack is energized or not as a function of whether or not current flows between the two aforementioned electrodes.
Ideally, the amount of current provided by the cathode should be adjustable. This is so sufficient current is generated to close the circuit, but not so much as to risk the health of aquarium inhabitants.
Two parameters related to the AC jack must be controllable as well. First, when the AC jack is energized, the length of time for which it is energized should be adjustable. Second, after the AC jack has been energized and then becomes de-energized, there must be a period of time during which the jack will not become energized again, even if the circuit involving the electrodes in the aquarium is closed again. This length of time during which the AC jack will not become energized after having been energized once should be adjustable.
Any device built to accomplish the above should be simple and use commonly available DC cells (i.e, AA, C, D).
* A description of protein skimmers can be found at:
http://reefkeeping.com/issues/2002-06/fm/feature/index.php