This is a siphon that has been used for some time in the aquarium / fish keeping industry. I learned about this type of siphon some time back, while viewing YouTube videos.
I will be using this particular siphon (overflow) as a skimmer, to remove particulates floating near the top of the water level. This overflow was built from 1/2" (13mm) PVC and fittings. I used a piece of 1" (21mm) PVC and a cap for the weir.
You can see the holes in the weir, allowing enough water to enter, regardless of the level, to allow the overflow to skim the top of the water without causing any danger to the fish.
As you can see, I (currently, and temporarily) have a 5 gallons (19 liters) capacity bucket for the filter / sump. I will be replacing this with the 15 gallons (57 liters) aquarium, as soon as I can get some glass cut to partition the tank into sections for different filter media, and a partition for the pump.
The pump can move considerably more water in the fish tank (barrel), than the 1" (21mm) SLO and the 1/2" (13mm) overflow can remove from the tank, combined. So, it isn't an issue using this tank to test experiments such as this. In fact, immediately below, you can see the 1/2" (13mm) return line (with the ball valve going back into the bucket), used to reduce the flow into the fish barrel and to help keep more water in the bucket (filter / sump).
I'm impressed with the performance of this overflow, even at 1/2" (13mm). What's really nice about the overflow is, if / when the power goes off, it automatically stops, just like the SLO does. When the power returns, the pump begins pumping again, and the overflow and SLO begin their respective siphoning actions. So, neither pipe will siphon all the water from the tank, leaving the fish high and dry - my first priority.
I get a lot of my filter and water flow ideas and information from the fellow in the video below, Joey Mullen. He has a lot of years in fish keeping. Here is one he made on his updated overflow.
Flow rates for various sizes of PVC, according to Joey, are below: