Question on Circulation between two tanks

[Dorkel Marine 1]

Ok rossb I'm slowly getting the feel for this system. A little confused about the hole to stop the syphon. Is it below the water. Be patient I'm not a mechanical person. I'm thinkin that if the hole is on the intake which is taken from the lower tank wouldn't the syphon never start since its above the water. Just wonderin. Like I said a little slow. Next question is how did you drill into the aqua clear. Hole saw? I was contemplatin using another smaller aqua clear for the intake. Thanks for bearing with me so far. I appreciate your help and photos.