For my saltwater tank I used one connected to my RODI to automatically top off. I used some scrap acrylic to make a L shaped bracket that attaches to the top of my sump and then goes into the water. I drilled a hole in that and put a cheap float valve on it that is similar to the one below. My sump is made of wood so I just screwed the bracket into the top of it. It's been a few years and so far I haven't had any problems with the valve.
But if this is for water changes and you're just worried about a drain clog, I'd argue a second drain slightly higher than the first is the safest thing possible.