New design? Float valve?

I'm new to salt water tanks, not fresh water however. I see that the popular method is a over flow, but that leaves a question, what about things floating mid level and lower? What about connecting a hinged valve to something that floats, it would allow water in when the sump refills the tank to a level, and closed when the sump has used enough water from the DT to lower the levels.

 

In comparison think how about a toilet works to detect when the water level is low.

http://t0.gstatic.com/images?q=tbn:ANd9GcTWJBO8eJI5yK_aM4o2haC0jfHS2UTri9Nt49pgJ_ZD7IQzhwUUEVutc7w

 

What do you mean things floating? in the tank? thell eventually either settle to the bottom, or you get some good power heads to keep the "things" suspended in the water coulumn where it has a chance of getting pushed into the sump, filtered, and returned.

Im not sure how you would go about what your describing, the way they work is just a continuous pump feeding water back to the tank, and then it gets to the overflow, and so on and so on.