Looking at your pic, it might be easier to use an overflow from your main tank to your reservoir and then just pump the water back up to the main tank. The water level in the main tank will stay constant while the water level in the sump fluctuates. Switches and valves complicate the function and increase your odds of something failing and flooding. There are a number of different designs for overflow systems that are pretty bullet proof when it comes to failures. Another addition is just to add a small hole in the line running from the pump to the main tank located just above the water level in your main take. This little hole will allow air in when the pump is shut off and prevent water from syphoning back thru the pump. This is much more reliable than a check valve.
On mine the small hole is positioned so that it is facing the tank wall. This prevents it from spraying water all over while the pump is on.
Not trying to say that all the neat electric tech isn't really cool, just less reliable than gravity controlled systems.
On mine the small hole is positioned so that it is facing the tank wall. This prevents it from spraying water all over while the pump is on.
Not trying to say that all the neat electric tech isn't really cool, just less reliable than gravity controlled systems.