Automated water changes

I was thinking of maybe an automated python style device, which connects permanently to the cold water line (under the sink), and the sink drain. The device would use electronically controlled valves. Then the tubing from the device would go to the tank. A water change cycle would consist of the drain valve opening then water source valve opening to siphon water from the tank. Then after a predetermined period the drain valve closes, and the source water is routed to the tank. I guess there would have to be a water level switch in the tank to tell the controller when to shut off the incoming water.
 
Another question. Do you have any plans for building a DIY dosing pump? It doesnt seem like it should be too complicated to build one.
 
If I use gravity feed overflow then there would be no need to siphon water from the tank, just add new water and whatever overflows overflows to the drain. I dont think Ill be able to do a gravity drain.
 
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