Yeah, thanks. This would would do a water change too. The amount of pressure drop to get these switches to switch state (or reset) is almost exactly the number of inches of water my tank loses on a 50% water change - by coincidence.
But if I used this just for water change, the auto-top-off capability goes away. And then you have to get involved with using Prime - so it's not really unattended. With the smaller changes there are lots of possibilities, like no prime at all due to the small amounts. Or having a small (20g?) aging tank hooked up to the house water supply.
Steve - 33g reef and a 180g planted in need of a re-scape.