Auto Top-off set-up
Decided to use a file storage bin. It holds about 10g of water and is roughly 22x13x12". Also fits perfectly under the prop tank.
The auto top-off is run by a mag2.
I hard plumbed the whole project w/ pvc. Added a T w/ ball valve to help slow down the rate of flow from water reservoir to sump by redirecting a bulk of the water pumped by the mag2 back into the water container.
Did this to prevent water moving out of the water container too quickly and exposing the mag2 to air. Also figured adding freshwater to a reef as slowly as possible would be a more ideal situation.
Made a hole at the top of the lid and used a slip union so I could easily connect and disconnect the plumbing from the pump.
pvc then heads for the sump tank
And releases top-off water at the very top of the sump tank to avoid any back-siphoning issues.
Here's the float switch from californiareefs.com that turns the mag2 on when drops 1/2" or so.