Good Photo...Just one comment.
It is important on this setup to do two things.
1) Make sure that the return line will break siphon. Either drill a hole in the line, just above the water line or set the return line elevation near the top water line so that once the water drops more than 1/2", the siphon will be broken.
2) Make sure that the overflow box on the outside has two chambers. Chamber #1 will be where the siphon tube dumps the water. Once this chamber is full, the water will overflow into the #2 chamber which will have the drain hole. The overflow level needs to be set at an elevation that is higher than the siphon intake level. Once the power goes off, tank will will drop until water no longer spills over into the inside overflow box. But the siphon will continue until the water elevation in both the internal and external boxes are equal. I have seen some DIY designs on the internet and this is something that is often neglected.
I have been looking into adding a sump on my system and these are a few of the things that I have picked up so far. The picture above is a great help for visualizing how the system works.
