This is also something I was thinking about but that means drilling another hole and I don't see this hole drilled into other systems so I was wondering how most people do it or if they just let it fly and hope it doesn't happen.Secondary overflow connecting to normal one, just above desire water level maybe. Just have it covered in mesh or something so fish don't jump down it
now i'm just playing devil's advocate here but what happens should you get a clog after the t section to the point where neither will allow flow?