I like the idea of a pump and a solenoid, but I'd consider arranging
it differently and use a flow through pump (ie, the pump would allow
siphoning when not active).

Connect the one tank to the other with the pump in the middle as
usual.  At the apex of the tube (Which has to be slightly higher than
the maximum level of either tank) put a T in with the solenoid
allowing air to enter the tube.

Diagram:
http://flickr.com/photos/adavis/2590441784/

Note that the solenoid with the air tube can reside inside one of the
tanks as long as the apex of the main tube is above the maximum of
both water levels.  This will make it so you don't need to move the
apex of the tube into the light.  Unsightly tubing could be hidden
with just one open end of a tube exposed somewhere.

Further, with a more complex control and monitoring scheme, you could
possibly use the pump to merely prime the siphon, and then when tank 2
is full enough open the solenoid to stop the flow.  Very low power
with a low power solenoid.  the solenoid can be very small relative to
the water flow and still stop the siphon action, and it could even be
a relatively cheap air solenoid.

If the inner tank is at a higher level then you'd merely use the pump
continuously.   This requires more information about the levels of the
tanks relative to each other, though, so it may not be practical
unless you can arrange them so one is always higher than the other.

The pump can be anywhere along the main line.  Regardless of your
solution you might consider putting a cheap float valve on the inside
tank in case the electronic level monitor fails, controls fail, or
other parts fail.  This would prevent flooding.

If you want to go very cheap, you can take the solenoids you migh find
that run on 120VAC and re-wind the coil for a lower voltage.  They
aren't terribly energy efficient, though, so you might still be using
a lot of power to overcome the friction and spring in the valve.

Also, note that the the typical sprinkler valve has a small solenoid
you might be able to use without too much modification:
http://www.spudtech.com/content.asp?id=21

-Adam

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