I tested mine extensively, and could not find a scenario where it would flood or fail to restart. The emergency overflow elbow (not shown in my pic) would come into play if the slots got plugged. So it was not necessary to have 2 overflows in mine.
And again, let me emphasize the importance of that airline tubing connection to the intake of the return pump. Without that, I would never trust it, and of course it makes restarting the siphon automatic. I also had an airline valve in the tubing that I could adjust or close if I wanted.
I do not have the inside tubes glued, so I can adjust for different heights or take apart for cleaning if I like. Of course anything outside of the tank water has to be glued
On mine, the outside T is a little high because I had to clear the top of my sump. If it is set lower, you can handle an even higher flow. But at 1.25 inch PVC, I had lots of flow regardless.
I was using this to connect my refugium to my sump, and just disconnect it (not run the return) if I want to use it as a QT. Worked great.
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